Bump cffi from 1.14.3 to 1.15.1

Bumps [cffi](http://cffi.readthedocs.org) from 1.14.3 to 1.15.1.

---
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

@@ -1,5 +1,5 @@
 certifi==2020.6.20
-cffi==1.14.5
+cffi==1.15.1
 chardet==4.0.0
 elasticsearch==7.17.5
 GDAL==3.2.2