Upgrade aiohttp to a minimum of 3.7.4

[bachya]

Describe what the PR does:

This PR upgrades aiohttp to 3.7.4, which fixes GHSA-v6wp-4m6f-gcjg.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #40 (633012b) into dev (7a9e8e9) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev       #40   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9        10    +1     
  Lines          163       164    +1     
=========================================
+ Hits           163       164    +1     

Impacted Files	Coverage Δ
aioflo/__init__.py	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7a9e8e9...633012b. Read the comment docs.

[codeclimate]

Code Climate has analyzed commit 633012b and detected 0 issues on this pull request.

View more on Code Climate.