Move Python version checking to a separate module

Add a new module with methods for checking the Python version.

Instead of handling Python3 imports with try...except blocks, first
check the python version and then import the relevant modules.  This
makes the code a bit cleaner and will result in less diff when/if we
remove support for Python < 3 later.

Use the same mechanism to handle `input` vs. `raw_input` and add
suppression of pylint warnings caused by redefinition of the built-in
method `input`.

Change-Id: Ia403e525b88d77640a741ac50382146e7d635924
Also-by: Chirayu Desai <cdesai@cyanogenmod.org>
Signed-off-by: Chirayu Desai <cdesai@cyanogenmod.org>

main.py

@@ -22,13 +22,12 @@ import optparse
 import os
 import sys
 import time
-try:
-  import urllib2
-except ImportError:
-  # For python3
+
+from pyversion import is_python3
+if is_python3():
   import urllib.request
 else:
-  # For python2
+  import urllib2
   urllib = imp.new_module('urllib')
   urllib.request = urllib2
 
@@ -50,10 +49,10 @@ from pager import RunPager
 
 from subcmds import all_commands
 
-try:
+if not is_python3():
+  # pylint:disable=W0622
   input = raw_input
-except NameError:
-  pass
+  # pylint:enable=W0622
 
 global_options = optparse.OptionParser(
                  usage="repo [-p|--paginate|--no-pager] COMMAND [ARGS]"