hghave: move from requiring the PYTHON3 env var to looking for `python3`

We're starting to write new utility scripts in Python 3, and I think
it's reasonable to expect `python3` to be something we should use if
it's present. I wrote the regex to specifically ban Python 3 prior to
Python 3.5 because we won't ever support that.

I moved the only test that was using $PYTHON3 to just using `python3`
directly.

Differential Revision: https://phab.mercurial-scm.org/D4290

tests/hghave.py

@@ -631,7 +631,7 b' def has_py3k():'
 
 @check("py3exe", "a Python 3.x interpreter is available")
 def has_python3exe():
-    return 'PYTHON3' in os.environ
+    return matchoutput('python3 -V', br'^Python 3.(5|6|7|8|9)')
 
 @check("py3pygments", "Pygments available on Python 3.x")
 def has_py3pygments():

tests/test-check-py3-compat.t

@@ -27,7 +27,7 b''
   > -X hgdemandimport/demandimportpy2.py \
   > -X hgext/fsmonitor/pywatchman \
   > -X mercurial/thirdparty/cbor \
-  > | sed 's|\\|/|g' | xargs $PYTHON3 contrib/check-py3-compat.py \
+  > | sed 's|\\|/|g' | xargs python3 contrib/check-py3-compat.py \
   > | sed 's/[0-9][0-9]*)$/*)/'
   contrib/python-zstandard/setup.py not using absolute_import
   contrib/python-zstandard/setup_zstd.py not using absolute_import
@@ -48,6 +48,6 b''
 
 #if py3exe py3pygments
   $ testrepohg files 'set:(**.py) and grep(pygments)' | sed 's|\\|/|g' \
-  > | xargs $PYTHON3 contrib/check-py3-compat.py \
+  > | xargs python3 contrib/check-py3-compat.py \
   > | sed 's/[0-9][0-9]*)$/*)/'
 #endif