Trac #30758: for python 3.6, universal_newlines=, not text=, must be used in run()

after #30740, `text=True` is still there, in the call to `run()`, and
also not supported by Python 3.6.
Cf.
https://docs.python.org/3/library/subprocess.html
{{{
Changed in version 3.7: Added the text parameter,
as a more understandable alias of universal_newlines.
Added the capture_output parameter.
}}}

as reported on [https://groups.google.com/d/msgid/sage-
release/8404a7d9-b906-4301-b891-59733da482cen%40googlegroups.com sage-
release]

URL: https://trac.sagemath.org/30758
Reported by: dimpase
Ticket author(s): Dima Pasechnik
Reviewer(s): Clemens Heuberger

Author: Release Manager

src/sage/features/__init__.py

@@ -373,7 +373,7 @@ def 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, stdout=PIPE, stderr=PIPE, text=True, check=True)
+            proc = run('sage-guess-package-system', shell=True, stdout=PIPE, stderr=PIPE, universal_newlines=True, check=True)
             _cache_package_systems = [PackageSystem(proc.stdout.strip())]
         except CalledProcessError:
             pass
@@ -440,11 +440,11 @@ def _spkg_installation_hint(self, spkgs, prompt, feature):
         system = self.name
         try:
             proc = run(f'sage-get-system-packages {system} {spkgs}',
-                       shell=True, stdout=PIPE, stderr=PIPE, text=True, check=True)
+                       shell=True, stdout=PIPE, stderr=PIPE, universal_newlines=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, stdout=PIPE, stderr=PIPE, text=True, check=True)
+            proc = run(command, shell=True, stdout=PIPE, stderr=PIPE, universal_newlines=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:')