Suggestion: Added PEP 484 type hints

[ThatXliner]

Description

This library needs type hints. I use MyPy, which complain because this library has no type stubs. You can easily make them with

System settings

• Operating System: macOS 11.2.1 (Big Sur 😬)
• Terminal in use: iTerm2 Build 3.4.4
• Python version: 3.6.13 (in virtual environment), System (/usr/bin/python3): 3.8.2
  - Halo version:
  - pip freeze output:
• Poetry lockfile: https://gist.github.com/ThatXliner/cc3ed9d05885a068e14db46ab006c4ee

Error

N/A

Expected behaviour

MyPy won't get angry

Steps to recreate

• Install MyPy
• Use MyPy (mypy .) on codebase that imports Halo
• MyPy complains:

...   # Other errors
foor/bar.py:N: error: Cannot find implementation or library stub for module named 'halo'
foo/bar.py:N: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
...  # Other errors

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