Improve Inheriting Workplane Fluent Methods (#677)

* Return Instances of the Type of self from Fluent Methods

* Updated `Workplane.workplane()`, `Workplane.copyWorkplane()`, `Workplane.newObject()` to return instances of a type that's based on the input parameters (such as `self`).
* Updated type hints to reflect that return types of fluent methods depend on the type of method arguments.
* Added a unit test to verify fluent methods in a derived class return instances of the derived class.

* Fix Workpane MyPy Errors

* Fixed type hints in `sweep()` and `_selectObjects()`.

* Fix `test_cadquery.py` Formatting

* Ran `black` on `test_cadquery.py`.

* Renamed `TypeVar` `WorkplaneT` to `T`

* Renamed `WorkplaneT` to `T` in `cq.py`.

* Reformat with Black

* Re-formatted `cq.py` with `black`.

* Re-format with the Right Version of Black (19.10b0)

* Re-formatted `cq.py` with `black` 19.10b0.

tests/test_cadquery.py

@@ -225,6 +225,27 @@ class TestCadQuery(BaseTest):
         self.assertEqual(26, s.faces().size())
         self.saveModel(s)
 
+    def testFluentMethodInheritance(self):
+        """
+        Tests that a derived class inherits fluent methods which return
+        instances of derived class when inherited.
+        """
+
+        class ExtendedWorkplane(Workplane):
+            def nonExistentInWorkplane(self):
+                pass
+
+        # Call an inherited fluent method:
+        wp = ExtendedWorkplane("XY").moveTo(1, 2)
+
+        # Verify that the inherited method returned an instance of the derived
+        # class:
+        self.assertEqual(type(wp), ExtendedWorkplane)
+
+        # The following is redundant, but can make the use case clearer.
+        # This must not raise an AttributeError:
+        wp.nonExistentInWorkplane()
+
     def testPointList(self):
         """
         Tests adding points and using them