build(deps): bump cffi from 1.12.3 to 1.15.0

Bumps [cffi](http://cffi.readthedocs.org) from 1.12.3 to 1.15.0.

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/base.txt

@@ -37,7 +37,7 @@ requests==2.25.1
 #       - urllib3
 cachetools==3.1.1
 certifi==2020.6.20
-cffi==1.12.3
+cffi==1.15.0
 chardet==4.0.0
 google-auth-httplib2==0.0.4
 httplib2==0.19.0