pycompat: make pycompat demandimport friendly

pycompat.py includes hack to import modules whose names are changed in Python 3.
We use try-except to load module according to the version of python. But this
method forces us to import the modules to raise an ImportError and hence making
it demandimport unfriendly.

This patch changes the try-except blocks to a single if-else block. To avoid
test-check-pyflakes.t complain about unused imports, pycompat.py is excluded
from the test.

mercurial/pycompat.py

@@ -10,53 +10,26 b' This contains aliases to hide python ver'
 
 from __future__ import absolute_import
 
-try:
+import sys
+
+if sys.version_info[0] < 3:
     import cPickle as pickle
-    pickle.dumps
-except ImportError:
-    import pickle
-    pickle.dumps # silence pyflakes
-
-try:
+    import cStringIO as io
     import httplib
-    httplib.HTTPException
-except ImportError:
-    import http.client as httplib
-    httplib.HTTPException
-
-try:
+    import Queue as _queue
     import SocketServer as socketserver
-    socketserver.ThreadingMixIn
-except ImportError:
-    import socketserver
-    socketserver.ThreadingMixIn
-
-try:
+    import urlparse
     import xmlrpclib
-    xmlrpclib.Transport
-except ImportError:
+else:
+    import http.client as httplib
+    import io
+    import pickle
+    import queue as _queue
+    import socketserver
+    import urllib.parse as urlparse
     import xmlrpc.client as xmlrpclib
-    xmlrpclib.Transport
-
-try:
-    import urlparse
-    urlparse.urlparse
-except ImportError:
-    import urllib.parse as urlparse
-    urlparse.urlparse
 
-try:
-    import cStringIO as io
     stringio = io.StringIO
-except ImportError:
-    import io
-    stringio = io.StringIO
-
-try:
-    import Queue as _queue
-    _queue.Queue
-except ImportError:
-    import queue as _queue
 empty = _queue.Empty
 queue = _queue.Queue
 

tests/test-check-pyflakes.t

@@ -6,7 +6,9 b''
 run pyflakes on all tracked files ending in .py or without a file ending
 (skipping binary file random-seed)
 
-  $ hg locate 'set:**.py or grep("^#!.*python")' 2>/dev/null \
+  $ hg locate 'set:**.py or grep("^#!.*python")' \
+  > -X mercurial/pycompat.py \
+  > 2>/dev/null \
   > | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   tests/filterpyflakes.py:61: undefined name 'undefinedname'