gh-120037: User site packages are being added when site module is enabled in _pth file

[tanzim]

Don't add user site package directory to sys.path when a distribution specific _.pth exists with site import enabled.

I'd prefer that we add some tests for this, however none of the embedding tests looks like they test with _pth files enabled?

Thanks to @eryksun for the fix suggestion.

• Issue: User site packages are being added when site module is enabled in _pth file #120037

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[eryksun]

Let's update _pthFileTests in "Lib/test/test_site.py" to verify that sys.flags.no_user_site is nonzero.

Basic test:

    @support.requires_subprocess()
    def test_underpth_no_user_site(self):
        pth_lines = [test.support.STDLIB_DIR, 'import site']
        exe_file = self._create_underpth_exe(pth_lines)
        p = subprocess.run([exe_file, '-X', 'utf8', '-c',
                            'import sys; '
                            'sys.exit(not sys.flags.no_user_site)'])
        self.assertEqual(p.returncode, 0, "sys.flags.no_user_site was 0")

[zooba]

Suggested change

	Fix user site packages directory being added to `sys.path` when site module is enabled in `._pth` file. Patch by Tanzim Husain.
	Fix user site packages directory being enabled when a ``._pth`` file has been used. Patch by Tanzim Husain.

This is a ReST file, not Markdown, so the syntax is slightly different. This should work to allow the docs builds to pass.

Edit And simplified the text a little.

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[zooba]

Seems like we're not getting a CLA, so I created #137428 to get this change in.