Bump aiohttp from 2.1.0 to 3.7.4 in /async_downloader

Bumps [aiohttp](https://github.com/aio-libs/aiohttp) from 2.1.0 to 3.7.4.
- [Release notes](https://github.com/aio-libs/aiohttp/releases)
- [Changelog](https://github.com/aio-libs/aiohttp/blob/master/CHANGES.rst)
- [Commits](aio-libs/aiohttp@v2.1.0...v3.7.4)

---
updated-dependencies:
- dependency-name: aiohttp
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

async_downloader/requirements.txt

@@ -1 +1 @@
-aiohttp==2.1.0
+aiohttp==3.7.4