tests: use pyflakes module instead of pyflakes executable

This results in that the pyflakes version specific to the configured Python
version is used.

This way, both the Python 2-specific and the Python 3-specific pyflakes are run
by the test suite (depending on the configured Python version).

For downstream projects which are using Mercurial’s test infrastructure and are
not yet ported to Python 3 (e.g. hg-git) it ensures that the correct pyflakes
version is run even when the system’s default pyflakes is the Python 3-specific
one.

tests/hghave.py

@@ -553,7 +553,7 b' def has_root():'
 @check("pyflakes", "Pyflakes python linter")
 def has_pyflakes():
     return matchoutput(
-        "sh -c \"echo 'import re' 2>&1 | pyflakes\"",
+        "sh -c \"echo 'import re' 2>&1 | $PYTHON -m pyflakes\"",
         br"<stdin>:1: 're' imported but unused",
         True,
     )

tests/test-check-pyflakes.t

@@ -20,7 +20,7 b' run pyflakes on all tracked files ending'
   > -X mercurial/thirdparty/concurrent \
   > -X mercurial/thirdparty/zope \
   > 2>/dev/null \
-  > | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
+  > | xargs $PYTHON -m pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   contrib/perf.py:*: undefined name 'xrange' (glob) (?)
   mercurial/hgweb/server.py:*: undefined name 'reload' (glob) (?)
   mercurial/util.py:*: undefined name 'file' (glob) (?)