Bump aiohttp from 1.3.1 to 3.7.4 in /examples/server/aiohttp

Bumps [aiohttp](https://github.com/aio-libs/aiohttp) from 1.3.1 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](https://github.com/aio-libs/aiohttp/compare/1.3.1...v3.7.4)

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

examples/server/aiohttp/requirements.txt

@@ -1,4 +1,4 @@
-aiohttp==1.3.1
+aiohttp==3.7.4
 async-timeout==1.1.0
 chardet==2.3.0
 multidict==2.1.4