Bump aiohttp from 3.7.3 to 3.7.4 in /python_backend/dnd (#24)

Bumps [aiohttp](https://github.com/aio-libs/aiohttp) from 3.7.3 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@v3.7.3...v3.7.4)

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python_backend/dnd/requirements.txt

@@ -1,4 +1,4 @@
-aiohttp==3.7.3
+aiohttp==3.7.4
 async-timeout==3.0.1
 attrs==20.3.0
 chardet==3.0.4