fix: fix launching Python 3.11 from <3.8 on macOS

On macOS, some older versions of Python leak __PYVENV_LAUNCHER__ into
the interpreter's environment variables. Starting with Python 3.11, this
causes problems with Python being able to start correctly when launched
from the Python bootstrap template. This change introduces a workaround
to prevent the problematic environment variable from leaking into
launched subprocesses.

I've verified this problem exists with the version of Python 3.7
included with Xcode, and there are reports of it also being an issue
with the Python 3.8 included with Xcode. Later versions of Python have
fixed this.

Context:
- https://issues.pigweed.dev/310293060
- xonsh/xonsh#3576 (comment)
- pypa/virtualenv#1458

python/private/python_bootstrap_template.txt

@@ -528,6 +528,13 @@ def Main():
   else:
     cov_tool = None
 
+  # Some older Python versions on macOS (namely Python 3.7) may unintentionally
+  # leave this environment variable set after starting the interpreter, which
+  # causes problems with Python subprocesses correctly locating sys.executable,
+  # which subsequently causes failure to launch on Python 3.11 and later.
+  if '__PYVENV_LAUNCHER__' in os.environ:
+    del os.environ['__PYVENV_LAUNCHER__']
+
   new_env.update((key, val) for key, val in os.environ.items() if key not in new_env)
 
   workspace = None