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.

Author: pavpen