tests: use venv on Python 3 stable

tests: use venv on Python 3

This test was failing in some Python 3 environments because
`$PYTHON -m virtualenv` was somehow resulting in Python 2
being used. Why, I'm not sure.

Python 3 includes virtualenv in the standard library as the
`venv` module.

This commit changes test-install.t to use `$PYTHON -m venv` on
Python 3 and `$PYTHON -m virtualenv` on Python 2 (if available).

I chose to make some test output duplicated because we can't
have nested conditionals and there is no easy way to express
ORing of hghave checks.

Differential Revision: https://phab.mercurial-scm.org/D7224

--- a/tests/test-install.t	Mon Nov 04 20:10:51 2019 -0800
+++ b/tests/test-install.t	Mon Nov 04 20:33:38 2019 -0800
@@ -236,20 +236,53 @@
 
 #endif
 
-#if virtualenv
-
 Verify that Mercurial is installable with pip. Note that this MUST be
 the last test in this file, because we do some nasty things to the
 shell environment in order to make the virtualenv work reliably.
 
+On Python 3, we use the venv module, which is part of the standard library.
+On Python 2, we use the 3rd party virtualenv module, if available.
+
   $ cd $TESTTMP
+  $ unset PYTHONPATH
+
+#if py3
+  $ "$PYTHON" -m venv installenv >> pip.log
+
+Note: we use this weird path to run pip and hg to avoid platform differences,
+since it's bin on most platforms but Scripts on Windows.
+  $ ./installenv/*/pip install --no-index $TESTDIR/.. >> pip.log
+  $ ./installenv/*/hg debuginstall || cat pip.log
+  checking encoding (ascii)...
+  checking Python executable (*) (glob)
+  checking Python version (3.*) (glob)
+  checking Python lib (*)... (glob)
+  checking Python security support (*) (glob)
+  checking Mercurial version (*) (glob)
+  checking Mercurial custom build (*) (glob)
+  checking module policy (*) (glob)
+  checking installed modules (*/mercurial)... (glob)
+  checking registered compression engines (*) (glob)
+  checking available compression engines (*) (glob)
+  checking available compression engines for wire protocol (*) (glob)
+  checking "re2" regexp engine \((available|missing)\) (re)
+  checking templates ($TESTTMP/installenv/*/site-packages/mercurial/templates)... (glob)
+  checking default template ($TESTTMP/installenv/*/site-packages/mercurial/templates/map-cmdline.default) (glob)
+  checking commit editor... (*) (glob)
+  checking username (test)
+  no problems detected
+#endif
+
+#if no-py3 virtualenv
+
 Note: --no-site-packages is deprecated, but some places have an
 ancient virtualenv from their linux distro or similar and it's not yet
 the default for them.
-  $ unset PYTHONPATH
+
   $ "$PYTHON" -m virtualenv --no-site-packages --never-download installenv >> pip.log
   DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. (?)
   DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support (?)
+
 Note: we use this weird path to run pip and hg to avoid platform differences,
 since it's bin on most platforms but Scripts on Windows.
   $ ./installenv/*/pip install --no-index $TESTDIR/.. >> pip.log
@@ -258,8 +291,7 @@
   $ ./installenv/*/hg debuginstall || cat pip.log
   checking encoding (ascii)...
   checking Python executable (*) (glob)
-  checking Python version (2.*) (glob) (no-py3 !)
-  checking Python version (3.*) (glob) (py3 !)
+  checking Python version (2.*) (glob)
   checking Python lib (*)... (glob)
   checking Python security support (*) (glob)
     TLS 1.2 not supported by Python install; network connections lack modern security (?)