Fix multi-part module import errors (#4395)

* Pyright detects and reports cases where a multi-part module name is accessed but is not explicitly imported. These are dangerous because they rely on import resolution ordering within a program, which can easily change. This change eliminates errors detected by pyright.

* Fixed regression caught by CI test.

* Fixed black formatting issues.

Co-authored-by: Eric Traut <erictr@microsoft.com>

stdlib/2and3/mailbox.pyi

@@ -1,4 +1,4 @@
-import email
+import email.message
 from _typeshed import AnyPath
 from types import TracebackType
 from typing import (