Improve type stubs (#1489)

* Add type stubs for GithubException subclasses

* Install type stubs per PEP 561

See https://mypy.readthedocs.io/en/stable/installed_packages.html#making-pep-561-compatible-packages.

* Make PaginatedList a generic type

* Add type stubs for __init__.py

* Add type stub for NamedUser.get_projects

* Add GithubObject superclasses to type stubs

Fixes #1490

github/GithubObject.pyi

@@ -8,22 +8,6 @@ from github.Organization import Organization
 from github.PullRequestReview import PullRequestReview
 from github.Requester import Requester
 
-class CompletableGithubObject:
-    def __eq__(self, other: Any) -> bool: ...
-    def __init__(
-        self,
-        requester: Requester,
-        headers: Dict[str, Union[str, int]],
-        attributes: Dict[str, Any],
-        completed: bool,
-    ) -> None: ...
-    def __ne__(self, other: Any) -> bool: ...
-    def _completeIfNeeded(self) -> None: ...
-    def _completeIfNotSet(
-        self, value: Union[_ValuedAttribute, _BadAttribute, _NotSetType]
-    ) -> None: ...
-    def update(self) -> bool: ...
-
 class GithubObject:
     def __init__(
         self,
@@ -107,9 +91,25 @@ class GithubObject:
     @classmethod
     def setCheckAfterInitFlag(cls, flag: bool) -> None: ...
 
-class NonCompletableGithubObject:
+class NonCompletableGithubObject(GithubObject):
     def _completeIfNeeded(self) -> None: ...
 
+class CompletableGithubObject(GithubObject):
+    def __eq__(self, other: Any) -> bool: ...
+    def __init__(
+        self,
+        requester: Requester,
+        headers: Dict[str, Union[str, int]],
+        attributes: Dict[str, Any],
+        completed: bool,
+    ) -> None: ...
+    def __ne__(self, other: Any) -> bool: ...
+    def _completeIfNeeded(self) -> None: ...
+    def _completeIfNotSet(
+        self, value: Union[_ValuedAttribute, _BadAttribute, _NotSetType]
+    ) -> None: ...
+    def update(self) -> bool: ...
+
 class _BadAttribute:
     def __init__(
         self, value: Any, expectedType: Any, exception: Optional[ValueError] = ...