Run mypy on GitHub actions (#7)

Automatically run mypy on GitHub actions (with the strict option enabled).

Author: AttackingOrDefending

.github/workflows/mypy.yml

@@ -0,0 +1,16 @@
+name: Mypy
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    name: Mypy
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: "3.10"
+    - run: |
+        pip install --upgrade pip
+        pip install numpy mypy
+        mypy --strict tictactoe

README.md

@@ -1,5 +1,9 @@
 # python-tictactoe
-[![PyPI version](https://badge.fury.io/py/python-tictactoe.svg)](https://badge.fury.io/py/python-tictactoe) [![Tests](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/tests.yml/badge.svg)](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/tests.yml) [![Build](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/build.yml/badge.svg)](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/build.yml) [![codecov](https://codecov.io/gh/AttackingOrDefending/python-tictactoe/branch/main/graph/badge.svg?token=7N5LHRA3OC)](https://codecov.io/gh/AttackingOrDefending/python-tictactoe)
+[![PyPI version](https://badge.fury.io/py/python-tictactoe.svg)](https://badge.fury.io/py/python-tictactoe)
+[![Tests](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/tests.yml/badge.svg)](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/tests.yml)
+[![Build](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/build.yml/badge.svg)](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/build.yml)
+[![Mypy](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/mypy.yml/badge.svg)](https://github.com/AttackingOrDefending/python-tictactoe/actions/workflows/mypy.yml)
+[![codecov](https://codecov.io/gh/AttackingOrDefending/python-tictactoe/branch/main/graph/badge.svg?token=7N5LHRA3OC)](https://codecov.io/gh/AttackingOrDefending/python-tictactoe)
 
 A tic-tac-toe library that supports different sizes and dimensions and how many marks in a row to win.
 
@@ -12,7 +16,7 @@ Download and install the latest release:
 
 ## Features
 
-* Includes strict mypy typings.
+* Includes mypy typings.
 
 * Different board sizes
 ```python