tests: mock aiohttp responses and fix windows

[onerandomusername]

fix: use WindowsSelectorEventLoopPolicy on windows

in addition, will use aioresponses for our test suite so no aiohttp sessions actually make real requests.

[codecov]

Codecov Report

Merging #125 (8fea42f) into main (4b07917) will decrease coverage by 0.22%.
The diff coverage is 50.00%.

❗ Current head 8fea42f differs from pull request most recent head 25192c5. Consider uploading reports for the commit 25192c5 to get more accurate results

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
- Coverage   70.90%   70.68%   -0.23%     
==========================================
  Files          33       33              
  Lines        1619     1634      +15     
  Branches      199      202       +3     
==========================================
+ Hits         1148     1155       +7     
- Misses        439      445       +6     
- Partials       32       34       +2     

Impacted Files	Coverage Δ
modmail/bot.py	37.87% <38.88%> (-0.97%)	⬇️
modmail/__init__.py	100.00% <100.00%> (ø)

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[onerandomusername]

Fixing conflicts and then merging.

[coveralls]

Pull Request Test Coverage Report for Build 1509624103

• 47 of 57 (82.46%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 78.756%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/test_fixtures.py	18	19	94.74%
modmail/bot.py	9	18	50.0%

Totals
Change from base Build 1509599375:	0.2%
Covered Lines:	1672
Relevant Lines:	2123

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💛 - Coveralls