mypy: Correct the story about skipping `zulip`.

Also remove the ignore_missing_imports on it, which has no effect.
That setting only does anything when mypy doesn't know where to find
the module; follow_imports only does anything when it does know.

See upstream docs:
  http://mypy.readthedocs.io/en/latest/config_file.html#per-module-flags

(I confirmed with `strace -f -efile` that the place mypy is finding
the `zulip` module is in fact in the virtualenv.)

mypy.ini

@@ -46,6 +46,26 @@ ignore_errors = True
 [mypy-zerver.tornado.autoreload]
 ignore_errors = True
 
+
+#
+#
+# SKIP IMPORTS
+#
+#
+
+[mypy-zulip]
+# mypy is finding this module in the virtualenv.  We should tell it
+# about it on the command line; until we do, everything imported from
+# it gets treated as Any, and we have to acknowledge it here.
+#
+# (The mypy daemon insists that everything it checks, it's told about
+# in advance, which means setting follow_imports to either `skip` or
+# `error`.  Our global setting of `error` gets us an error if there's
+# something it *could* check, because it can find the file, but we
+# haven't told it to.)
+follow_imports = skip
+
+
 #
 #
 # IGNORE MISSING IMPORTS
@@ -66,15 +86,6 @@ ignore_missing_imports = True
 # Our own libraries, maintained outside this repo and installed
 # through pip.  We need to tell mypy where to find them.
 
-[mypy-zulip]
-ignore_missing_imports = True
-# This isn't the right solution (we should tell mypy where to find
-# them), but it's necessary to avoid warnings like this:
-#     note: Import of 'zulip' ignored
-# I'm not sure how to tell mypy where to find these without having it
-# also process everything else in the Zulip virtualenv...
-follow_imports = skip
-
 [mypy-zulip_bots.lib]
 ignore_missing_imports = True