Added support for python before 3.9

[avideci]

Python 3.8.10
[GCC 9.4.0] on linux

>>> import tiktoken
>>> tiktoken
<module 'tiktoken' from 'venv/lib/python3.8/site-packages/tiktoken/__init__.py'>
>>> tiktoken.get_encoding('gpt2')
<Encoding 'gpt2'>

See discussion in #1

[avideci]

Please let me know which formatter you are using (black?)
I'll sort the imports and reformat the code as you wish.

[hauntsaninja]

Thanks for the PR! I just released tiktoken 0.1.2 that adds support for Python 3.8

Note that I'm not planning on adding support for 3.7, see #1
This PR doesn't work for 3.7 as we use some other 3.8 features (like cached_property and walrus)