tests: stop excluding the pycompat module from pyflakes

I assume this was skipped because of all of the py2 stuff causing a lot of spew.
The "unused" imports are left in place in case any 3rd party stuff is using it.
I don't care about most of it, but TortoiseHg uses `io` and `queue`, so
minimally I'd like to keep those.

Differential Revision: https://phab.mercurial-scm.org/D12423

tests/test-check-pyflakes.t

@@ -15,11 +15,19 b' run pyflakes on all tracked files ending'
 
   $ testrepohg locate 'set:**.py or grep("^#!.*python")' \
   > -X hgext/fsmonitor/pywatchman \
-  > -X mercurial/pycompat.py -X contrib/python-zstandard \
+  > -X contrib/python-zstandard \
   > -X mercurial/thirdparty \
   > 2>/dev/null \
   > | 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/pycompat.py:*:* 'codecs' imported but unused (glob)
+  mercurial/pycompat.py:*:* 'concurrent.futures' imported but unused (glob)
+  mercurial/pycompat.py:*:* 'http.client as httplib' imported but unused (glob)
+  mercurial/pycompat.py:*:* 'http.cookiejar as cookielib' imported but unused (glob)
+  mercurial/pycompat.py:*:* 'io' imported but unused (glob)
+  mercurial/pycompat.py:*:* 'queue' imported but unused (glob)
+  mercurial/pycompat.py:*:* 'socketserver' imported but unused (glob)
+  mercurial/pycompat.py:*:* 'xmlrpc.client as xmlrpclib' imported but unused (glob)
   mercurial/util.py:*:* 'pickle' imported but unused (glob)