Update cffi requirement from ~=1.15.1 to ~=1.16.0

Updates the requirements on [cffi](https://github.com/python-cffi/cffi) to permit the latest version.
- [Release notes](https://github.com/python-cffi/cffi/releases)
- [Commits](python-cffi/cffi@v1.15.1...v1.16.0)

---
updated-dependencies:
- dependency-name: cffi
  dependency-type: direct:production
...

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

requirements.txt

@@ -19,7 +19,7 @@ bleach~=6.0.0
 broadbean==0.11.0
 cachetools~=5.3.0
 certifi~=2023.7.22
-cffi~=1.15.1
+cffi~=1.16.0
 charset-normalizer~=3.2.0
 click~=8.1.3
 click-default-group~=1.2.2