Add flynt to lint (to enforce f-string use)

Author: akx

dev_requirements.txt

@@ -1,4 +1,5 @@
 flake8>=3.9.2
+flynt~=0.69.0
 pytest==6.2.5
 pytest-timeout==2.0.1
 tox==3.24.4

tox.ini

@@ -132,6 +132,7 @@ docker =
 commands =
     flake8 --max-line-length=88
     vulture redis whitelist.py --min-confidence 80
+    flynt --fail-on-change --dry-run .
 skipsdist = true
 skip_install = true