Bump aiohttp from 3.7.3 to 3.9.0

Bumps [aiohttp](https://github.com/aio-libs/aiohttp) from 3.7.3 to 3.9.0.
- [Release notes](https://github.com/aio-libs/aiohttp/releases)
- [Changelog](https://github.com/aio-libs/aiohttp/blob/master/CHANGES.rst)
- [Commits](aio-libs/aiohttp@v3.7.3...v3.9.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -1,4 +1,4 @@
-aiohttp==3.7.3; python_version >= "3.6"
+aiohttp==3.9.0; python_version >= "3.6"
 appdirs==1.4.4; python_full_version >= "3.6.1" and python_full_version < "4.0.0"
 async-timeout==3.0.1; python_full_version >= "3.5.3" and python_version >= "3.6"
 attrs==20.3.0; python_version >= "3.6" and python_full_version < "3.0.0" or python_full_version >= "3.4.0" and python_version >= "3.6"