Bump cffi from 1.14.6 to 1.15.0

Bumps [cffi](http://cffi.readthedocs.org) from 1.14.6 to 1.15.0.

---
updated-dependencies:
- dependency-name: cffi
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements.txt

@@ -14,7 +14,7 @@ async-timeout==3.0.1
     # via aiohttp
 attrs==21.2.0
     # via aiohttp
-cffi==1.14.6
+cffi==1.15.0
     # via pycares
 chardet==4.0.0
     # via aiohttp