gh-40490: Replace 'sage-python' with 'python3' in various scripts for Meson
    
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In a pure meson build there is no `sage-python`, so scripts declaring
using `sage-python` are not working in this context. To remedy this,
simply `python3` is used instead.

### 📝 Checklist

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- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: #40490
Reported by: Tobias Diez
Reviewer(s): Antonio Rojas, Dima Pasechnik

Author: Release Manager

src/bin/math-readline

@@ -1,15 +1,17 @@
-#!/usr/bin/env sage-python
+#!/usr/bin/env python3
 
 # Cleverly run Mathematica with the benefit of readline, which
 # is something the usual commercial mathematica doesn't provide!
 # See
 #    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/363500
 
-import sys
 import signal
 import subprocess
+import sys
+
 from sage.cpython.string import str_to_bytes
 
+
 def child_exited(*args):
     global child
     status = child.poll()

src/bin/sage

@@ -944,7 +944,7 @@ fi
 
 if [ "$1" = '-fixdistributions' -o "$1" = '--fixdistributions' ]; then
     shift
-    exec sage-python -m sage.misc.package_dir "$@"
+    exec python3 -m sage.misc.package_dir "$@"
 fi
 
 if [ "$1" = '-tox' -o "$1" = '--tox' ]; then
@@ -1012,7 +1012,7 @@ if [ "$1" = "-docbuild" -o "$1" = "--docbuild" ]; then
     # Redirect stdin from /dev/null. This helps with running TeX which
     # tends to ask interactive questions if something goes wrong. These
     # cause the build to hang. If stdin is /dev/null, TeX just aborts.
-    exec sage-python -m sage_docbuild "$@" </dev/null
+    exec python3 -m sage_docbuild "$@" </dev/null
 fi
 
 #####################################################################
@@ -1047,7 +1047,7 @@ if [ "$1" = '-gdb' -o "$1" = "--gdb" ]; then
         gdb --eval-command "run" \
             -args python "${SELF}-ipython" "$@" -i
     else
-        sage_dir=$(sage-python -c 'import os, sage; print(os.path.dirname(sage.__file__))')
+        sage_dir=$(python3 -c 'import os, sage; print(os.path.dirname(sage.__file__))')
         cygdb "$sage_dir" "$SAGE_SRC/sage" \
             -- --eval-command "run" \
             -args python "${SELF}-ipython" "$@" -i

src/bin/sage-cachegrind

@@ -21,5 +21,5 @@ else
     echo $CACHEGRIND_FLAGS
 fi
 
-valgrind --tool=cachegrind $CACHEGRIND_FLAGS sage-python -i
+valgrind --tool=cachegrind $CACHEGRIND_FLAGS python3 -i
 

src/bin/sage-cleaner

@@ -1,4 +1,4 @@
-#!/usr/bin/env sage-python
+#!/usr/bin/env python3
 
 #*****************************************************************************
 # This is the sage monitor *daemon*, which cleans up after Sage.
@@ -17,8 +17,14 @@
 #*****************************************************************************
 
 
-import os, shutil, sys, time, socket, errno, signal, atexit
-
+import atexit
+import errno
+import os
+import shutil
+import signal
+import socket
+import sys
+import time
 
 HOSTNAME = os.environ.get('HOSTNAME', socket.gethostname())
 DOT_SAGE = os.environ['DOT_SAGE']
@@ -30,6 +36,7 @@ pidfile = os.path.join(SAGE_TMP_ROOT, 'cleaner.pid')
 old_pidfile = os.path.join(DOT_SAGE, 'temp', 'cleaner-%s.pid'%HOSTNAME)
 
 import logging
+
 logger = logging.getLogger(__name__)
 
 

src/bin/sage-eval

@@ -1,8 +1,9 @@
-#!/usr/bin/env sage-python
+#!/usr/bin/env python3
 
 import sys
+
 from sage.all import *
-from sage.calculus.predefined import x
+from sage.calculus.predefined import x  # noqa: F401
 from sage.repl.preparse import preparse
 
 if len(sys.argv) > 1:

src/bin/sage-fixdoctests

@@ -1,4 +1,4 @@
-#!/usr/bin/env sage-python
+#!/usr/bin/env python3
 """
 Given the output of doctest and a file, adjust the doctests so they won't fail.
 

src/bin/sage-ipython

@@ -1,4 +1,4 @@
-#!/usr/bin/env sage-python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 """
 Sage IPython startup script.
@@ -7,6 +7,7 @@ Sage IPython startup script.
 # Display startup banner. Do this before anything else to give the user
 # early feedback that Sage is starting.
 from sage.misc.banner import banner
+
 banner()
 
 from sage.repl.interpreter import SageTerminalApp

src/bin/sage-list-packages

@@ -1,4 +1,4 @@
-#!/usr/bin/env sage-python
+#!/usr/bin/env python3
 r"""
 Script to list the Sage packages
 

src/bin/sage-massif

@@ -21,5 +21,5 @@ else
     echo $MASSIF_FLAGS
 fi
 
-valgrind --tool=massif $MASSIF_FLAGS sage-python -i
+valgrind --tool=massif $MASSIF_FLAGS python3 -i
 

src/bin/sage-notebook

@@ -1,20 +1,17 @@
-#!/usr/bin/env sage-python
+#!/usr/bin/env python3
 # -*- coding: utf-8; mode: python -*-
 
-import os
-import sys
-import ast
 import argparse
 import logging
-import textwrap
+import os
+import sys
 from contextlib import contextmanager
 
 logging.basicConfig()
 logger = logging.getLogger()
 
 from sage.misc.banner import banner
 
-
 _system_jupyter_url = "https://doc.sagemath.org/html/en/installation/launching.html#setting-up-sagemath-as-a-jupyter-kernel-in-an-existing-jupyter-notebook-or-jupyterlab-installation"
 
 
@@ -188,7 +185,9 @@ def sage_doc_server():
     from functools import partial
     from http.server import SimpleHTTPRequestHandler, ThreadingHTTPServer
     from threading import Thread
-    from sage.env import SAGE_DOC, SAGE_DOC_LOCAL_PORT as port
+
+    from sage.env import SAGE_DOC
+    from sage.env import SAGE_DOC_LOCAL_PORT as port
 
     server = ThreadingHTTPServer(('127.0.0.1', int(port)),
         partial(SimpleHTTPRequestHandler, directory=SAGE_DOC))