Use latest Meson prerelease for 'meson format'

Formatting rules change over time, and CI uses the latest Meson
prerelease, so we should match that version on developers' systems.  If
not running in CI, install Meson into a virtualenv that we update at most
once per day, and use it for format checks in sanity_checks.py and
format.py.

Author: bgilbert

.gitignore

@@ -2,3 +2,5 @@ __pycache__
 .idea
 .vscode
 *~
+
+tools/mesonenv

tools/utils.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 from contextlib import contextmanager
+import functools
 import operator
 import re
 import os
@@ -22,6 +23,8 @@
 from pathlib import Path
 import platform
 import subprocess
+import time
+import venv
 
 # a helper class which implements the same version ordering as RPM
 class Version:
@@ -129,13 +132,46 @@ def is_msys() -> bool:
 def is_macos():
     return any(platform.mac_ver()[0])
 
+@functools.cache
+def venv_meson_path() -> Path:
+    if os.getenv('CI', 'false') == 'true':
+        # assume CI already has a current Meson
+        return Path('meson')
+
+    env_dir = Path(__file__).parent / 'mesonenv'
+    if not env_dir.exists():
+        venv.create(env_dir, with_pip=True)
+
+    if platform.system() == 'Windows':
+        for subdir in 'Scripts', 'bin':
+            if (env_dir / subdir).exists():
+                meson = env_dir / subdir / 'meson.exe'
+                break
+        else:
+            raise Exception("Couldn't find venv bin dir")
+    else:
+        meson = env_dir / 'bin/meson'
+
+    try:
+        if meson.stat().st_mtime + 86400 >= time.time():
+            return meson
+    except FileNotFoundError:
+        pass
+
+    subprocess.run([
+        meson.with_stem('pip'), 'install', '--disable-pip-version-check',
+        '-qU', '--pre', 'meson'
+    ], check=True)
+    os.utime(meson)
+    return meson
+
 class FormattingError(Exception):
     pass
 
 def format_meson(files: T.Iterable[Path], *, check: bool = False) -> None:
     if not files:
         return
-    cmd: list[str | Path] = ['meson', 'format', '--configuration', './meson.format']
+    cmd: list[str | Path] = [venv_meson_path(), 'format', '--configuration', './meson.format']
     if check:
         cmd.append('--check-only')
     else: