Replace requests with aiohttp

[cooperlees]

• Change Master to use an aiohttp ClientSession
• Change Master.get method into an async generator to maintain API style
• Change Package class to use Master.get async generator
• Update utils user_agent() to always include aiohttp version + update tests
• Make verify.py use utils common USER_AGENT

Fix most tests + add in pytest-asyncio

• Happy to take the small % of coverage hit as I think I've covered most important parts, especially with the INTEGRATION run we do.

Fixes #6

[codecov]

Codecov Report

Merging #440 into master will decrease coverage by 0.6%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
- Coverage   82.44%   81.84%   -0.61%     
==========================================
  Files          10       10              
  Lines        1128     1151      +23     
  Branches      175      177       +2     
==========================================
+ Hits          930      942      +12     
- Misses        159      166       +7     
- Partials       39       43       +4

Impacted Files	Coverage Δ
src/bandersnatch/__init__.py	100% <ø> (ø)	⬆️
src/bandersnatch/verify.py	51.47% <0%> (-0.36%)	⬇️
src/bandersnatch/utils.py	95.5% <100%> (+4.49%)	⬆️
src/bandersnatch/main.py	76.82% <39.13%> (+4.1%)	⬆️
src/bandersnatch/mirror.py	86.01% <75%> (-0.84%)	⬇️
src/bandersnatch/package.py	81.2% <86.36%> (-4.24%)	⬇️
src/bandersnatch/master.py	84.26% <89.74%> (-0.46%)	⬇️

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[cooperlees]

I don't think anyone will ever review this. Going to merge and move forward doing the remaining 4.0 PRs and get an alpha release out maybe this week.

[techalchemy]

I'll make sure to swap my deployments to use the alpha