Bump aiohttp from 3.6.2 to 3.7.4 in /libraries/botbuilder-adapters-slack (#1553)

Bumps [aiohttp](https://github.com/aio-libs/aiohttp) from 3.6.2 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@v3.6.2...v3.7.4)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: tracyboehrer <tracyboehrer@users.noreply.github.com>

Author: dependabot[bot]

libraries/botbuilder-adapters-slack/requirements.txt

@@ -1,4 +1,4 @@
-aiohttp==3.6.2
+aiohttp==3.7.4
 pyslack
 botbuilder-core==4.13.0
 slackclient