Let's add type hints to PrettyTable.

The good news is much of the work is already done because typeshed has .pyi signature stubs:

• https://github.com/python/typeshed/tree/master/stubs/prettytable/prettytable

And here are some resources for automating the conversion of stubs into inline annotations:

• https://typing.readthedocs.io/en/latest/#type-hint-and-stub-integration

Tasks:

• Add this to .pre-commit-config.yaml so pre-commit adds this import to Python files and we can use newer typing Syntax on older Python 3.7:

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 3dbc9c0..316e7f5 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -15,6 +15,7 @@ repos:
     rev: 5.10.1
     hooks:
       - id: isort
+        args: [--add-import=from __future__ import annotations]
 
   - repo: https://github.com/PyCQA/flake8
     rev: 5.0.4

• Apply the type hints from typeshed to this repo
• Add mypy via pre-commit and make mypy pass (can be follow-up PR if there's much to do)
• Add py.typed and "Typing :: Typed" Trove classifier