convert: fix the RevisionSpec import in the bzr module

This import was failing (because its invalid) which was resulting in
the following tests failing with chg running:

 - test-convert-bzr.t
 - test-convert-bzr-directories.t
 - test-convert-bzr-ghosts.t
 - test-convert-bzr-treeroot.t
 - test-convert-bzr-114.t
 - test-convert-bzr-merges.t

This commit fixes the import which in turn fixes the tests.

Test Plan:
Ran the aforementioned tests with and without '--chg' option.

Differential Revision: https://phab.mercurial-scm.org/D936

hgext/convert/bzr.py

@@ -30,11 +30,12 b' try:'
     import bzrlib.bzrdir
     import bzrlib.errors
     import bzrlib.revision
-    import bzrlib.revisionspec.RevisionSpec
+    import bzrlib.revisionspec
     bzrdir = bzrlib.bzrdir
     errors = bzrlib.errors
     revision = bzrlib.revision
     revisionspec = bzrlib.revisionspec
+    revisionspec.RevisionSpec
 except ImportError:
     pass