Bump cffi from 1.14.6 to 1.15.0 (#53)

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>

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

requirements.txt

@@ -5,7 +5,7 @@ backports.entry-points-selectable==1.1.1
 bcrypt==3.2.0
 cachetools==4.2.4
 certifi==2021.10.8
-cffi==1.14.6
+cffi==1.15.0
 cfgv==3.3.1
 chardet==4.0.0
 charset-normalizer==2.0.9