Export IncompletableObject in the github namespace (#1450)

When IncompletableObject was added, it was only added to
GithubException, and not imported directly into the github namespace
like other exceptions. Correct that, and clean up the resultant now
unrequired import.

github/__init__.py

@@ -46,6 +46,7 @@ from .GithubException import (  # type: ignore
     BadCredentialsException,
     BadUserAgentException,
     GithubException,
+    IncompletableObject,
     RateLimitExceededException,
     TwoFactorException,
     UnknownObjectException,
@@ -73,6 +74,7 @@ __all__ = [
     Github,
     GithubException,
     GithubIntegration,
+    IncompletableObject,
     InputFileContent,
     InputGitAuthor,
     InputGitTreeElement,