* Start deprecation of Python 3.6

* Removed dependency on attrs and replaced with dataclasses

* Fix typing

* Added comments to assist with dropping support of Python versions in the future.

---------

Co-authored-by: Kevin Van Brunt <kmvanbrunt@gmail.com>

* Use latest version of mypy and fix type hinting accordingly

Also:
- Update Pipfile to never require mock since we only support Python 3.6+
- Remove Azure Pipelines badge from Readme and fix section links there
- Added an "inv format" task to run black and isort to auto-format all code before a commit

* Try to fix type errors on versions prior to 3.8

* Restored correct types in argparse_custom.py

Added support for 24-bit/RGB colors with the cmd2.RgbFg and cmd2.RgbBg classes.

Removed dependency on colorama.

Deprecated cmd2.fg. Use cmd2.Fg instead.
Deprecated cmd2.bg. Use cmd2.Bg instead.

Changed type of ansi.allow_style from a string to an ansi.AllowStyle Enum class.
Fixed bug where using choices on a Settable didn't verify that a valid choice had been entered.

  behavior of `ArgparseCompleter`.
* New function `register_argparse_argument_parameter()` allows developers to specify custom
  parameters to be passed to the argparse parser's `add_argument()` method. These parameters will
  become accessible in the resulting argparse Action object when modifying `ArgparseCompleter` behavior.