Enable python 3.8

[waghanza]

Hi @gi0baro,

In order to add emmett (aka weppy) in https://github.com/the-benchmarker/web-frameworks, it needs to be runnable on python 3.8

:information_source: python 3.8 is the current stable :information_source:

This PR :

• [x] Use html.escape instead of cgi.escape, since this was removed

@see https://docs.python.org/3/whatsnew/3.8.html#api-and-feature-removals

• [x] Test code base against python 3.8
• [x] Add python 3.8 classifier
• [x] Specify uvloop to be at minimum 0.14, 0.13 is not suitable for python 3.8
• [x] Specify digestmod, since required on python 3.8

@see https://docs.python.org/3/library/hmac.html#hmac.new

Regards,

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Closes #251

[gi0baro]

@waghanza thank you!