Trac #30740: Fix sage.feature.package_systems for Python 3.6

Follow-up from #30606.

As reported in https://groups.google.com/d/msg/sage-
release/0MnZQYItCYc/yhXDenfMAAAJ:

{{{
line 376, in package_systems
     proc = run('sage-guess-package-system', shell=True,
capture_output=True,
text=True, check=True)
   File "/usr/lib/python3.6/subprocess.py", line 423, in run
     with Popen(*popenargs, **kwargs) as process:
TypeError: __init__() got an unexpected keyword argument
'capture_output'
Makefile:190: recipe for target 'ptestlong' failed
}}}

So apparently my system python3 is used which is Python 3.6.9. The
option
capture_output has been introduced in python 3.7.

Blocker because this ticket fixes the broken doctest machinery on major
supported platforms including `ubuntu-bionic`

URL: https://trac.sagemath.org/30740
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Volker Braun

Author: Release Manager

src/sage/features/__init__.py

@@ -366,14 +366,14 @@ def package_systems():
         [Feature('homebrew'), Feature('sage_spkg'), Feature('pip')]
     """
     # The current implementation never returns more than one system.
-    from subprocess import run, CalledProcessError
+    from subprocess import run, CalledProcessError, PIPE
     global _cache_package_systems
     if _cache_package_systems is None:
         _cache_package_systems = []
         # Try to use scripts from SAGE_ROOT (or an installation of sage_bootstrap)
         # to obtain system package advice.
         try:
-            proc = run('sage-guess-package-system', shell=True, capture_output=True, text=True, check=True)
+            proc = run('sage-guess-package-system', shell=True, stdout=PIPE, stderr=PIPE, text=True, check=True)
             _cache_package_systems = [PackageSystem(proc.stdout.strip())]
         except CalledProcessError:
             pass
@@ -435,16 +435,16 @@ def _spkg_installation_hint(self, spkgs, prompt, feature):
             sage: fedora.spkg_installation_hint('openblas')  # optional - SAGE_ROOT
             'To install openblas using the fedora package manager, you can try to run:\n!sudo yum install openblas-devel'
         """
-        from subprocess import run, CalledProcessError
+        from subprocess import run, CalledProcessError, PIPE
         lines = []
         system = self.name
         try:
             proc = run(f'sage-get-system-packages {system} {spkgs}',
-                       shell=True, capture_output=True, text=True, check=True)
+                       shell=True, stdout=PIPE, stderr=PIPE, text=True, check=True)
             system_packages = proc.stdout.strip()
             print_sys = f'sage-print-system-package-command {system} --verbose --sudo --prompt="{prompt}"'
             command = f'{print_sys} update && {print_sys} install {system_packages}'
-            proc = run(command, shell=True, capture_output=True, text=True, check=True)
+            proc = run(command, shell=True, stdout=PIPE, stderr=PIPE, text=True, check=True)
             command = proc.stdout.strip()
             if command:
                 lines.append(f'To install {feature} using the {system} package manager, you can try to run:')