typing: add an assertion to the upgrade engine to help pytype stable

typing: add an assertion to the upgrade engine to help pytype

Differential Revision: https://phab.mercurial-scm.org/D10180

--- a/mercurial/upgrade_utils/engine.py	Thu Mar 11 19:21:58 2021 -0500
+++ b/mercurial/upgrade_utils/engine.py	Thu Mar 11 19:50:14 2021 -0500
@@ -516,6 +516,7 @@
     # reference to its new location. So clean it up manually. Alternatively, we
     # could update srcrepo.svfs and other variables to point to the new
     # location. This is simpler.
+    assert backupvfs is not None  # help pytype
     backupvfs.unlink(b'store/lock')
 
     return backuppath