Bump aiohttp-session from 2.7.0 to 2.8.0

Bumps [aiohttp-session](https://github.com/aio-libs/aiohttp_session) from 2.7.0 to 2.8.0.
- [Release notes](https://github.com/aio-libs/aiohttp_session/releases)
- [Changelog](https://github.com/aio-libs/aiohttp-session/blob/master/CHANGES.txt)
- [Commits](https://github.com/aio-libs/aiohttp_session/compare/v2.7.0...v2.8.0)

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

requirements-dev.txt

@@ -7,7 +7,7 @@ pytest-mock==1.10.4
 coverage==4.5.4
 sphinx==2.2.0
 pep257==0.7.0
-aiohttp-session==2.7.0
+aiohttp-session==2.8.0
 aiopg[sa]==0.16.0
 aioredis==1.2.0
 hiredis==1.0.0