global: use python3 in shebangs Python 3 is the future. We want Python scripts to be using Python 3 by default. This change updates all `#!/usr/bin/env python` shebangs to use `python3`. Does this mean all scripts use or require Python 3: no. In the test environment, the `PATH` environment variable in tests is updated to guarantee that the Python executable used to run run-tests.py is used. Since test scripts all now use `#!/usr/bin/env python3`, we had to update this code to install a `python3` symlink instead of `python`. It is possible there are some random scripts now executed with the incorrect Python interpreter in some contexts. However, I would argue that this was a pre-existing bug: we should almost always be executing new Python processes using the `sys.executable` from the originating Python script, as `python` or `python3` won't guarantee we'll use the same interpreter. Differential Revision: https://phab.mercurial-scm.org/D9273

--- a/tests/test-run-tests.t	Mon Nov 09 09:58:44 2020 -0800
+++ b/tests/test-run-tests.t	Fri Nov 06 13:58:59 2020 -0800
@@ -1412,7 +1412,7 @@
   > This should print the start of check-code. If this passes but the
   > previous check failed, that means we found a copy of check-code at whatever
   > RUNTESTSDIR ended up containing, even though it doesn't match TESTDIR.
-  >   $ head -n 3 "\$RUNTESTDIR"/../contrib/check-code.py | sed 's@.!.*python@#!USRBINENVPY@'
+  >   $ head -n 3 "\$RUNTESTDIR"/../contrib/check-code.py | sed 's@.!.*python3@#!USRBINENVPY@'
   >   #!USRBINENVPY
   >   #
   >   # check-code - a style and portability checker for Mercurial