WIP: Travis CI: Lower flake8 max-complexity to 15

Work In Progress Draft: Do not merge if test are not passing!!

Related to #2128, find all files that contain algorithms with > 15% McCabe complexity.

Author: cclauss

Diff for: .travis.yml

@@ -10,7 +10,7 @@ notifications:
     on_success: never
 before_script:
   - black --check . || true
-  - flake8 --ignore=E203,W503 --max-complexity=25 --max-line-length=88 --statistics --count .
+  - flake8 --ignore=E203,W503 --max-complexity=15 --max-line-length=88 --statistics --count .
   - scripts/validate_filenames.py  # no uppercase, no spaces, in a directory
   - pip install -r requirements.txt  # fast fail on black, flake8, validate_filenames
 script: