bzr: only support bzr conversion on Python3

Python 2 support will be dropped from Mercurial before anyone needs to do a
bzr -> hg conversion on Python 2 again. Bazaar tests were broken with the new
lib anyway, which we'll get into in the next patches.

Differential Revision: https://phab.mercurial-scm.org/D10512

tests/hghave.py

@@ -29,7 +29,8 b' except ImportError:'
 stdout = getattr(sys.stdout, 'buffer', sys.stdout)
 stderr = getattr(sys.stderr, 'buffer', sys.stderr)
 
-if sys.version_info[0] >= 3:
+is_not_python2 = sys.version_info[0] >= 3
+if is_not_python2:
 
     def _sys2bytes(p):
         if p is None:
@@ -169,6 +170,8 b' def has_baz():'
 
 @check("bzr", "Canonical's Bazaar client")
 def has_bzr():
+    if not is_not_python2:
+        return False
     try:
         import bzrlib
         import bzrlib.bzrdir