Assert that "version" file was a normal file

This satisfies typecheckers, which are worried about the case where
extractfile returns None. That doesn't bother me too much, since if it
happens it's an error condition anyway. This will make it so
non-optimized runs will get a somewhat nicer message in the unlikely
event that this happens.

Author: pganssle

update.py

@@ -139,6 +139,8 @@ def find_latest_version() -> str:
     fobj = io.BytesIO(r.content)
     with tarfile.open(fileobj=fobj, mode="r:gz") as tf:
         vfile = tf.extractfile("version")
+
+        assert vfile is not None, "version file is not a regular file"
         version = vfile.read().decode("utf-8").strip()
 
     assert re.match("\d{4}[a-z]$", version), version