Added flake8 to pre-commit config and set flake8 to ignore E203

E203 needs to be ignore due to a false positive alert for slicing. See. PyCQA/pycodestyle#373 (comment)

Author: s-weigand

.pre-commit-config.yaml

@@ -12,3 +12,8 @@ repos:
     hooks:
       - id: black
         language_version: python3
+
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.8.4
+    hooks:
+      - id: flake8

setup.cfg

@@ -12,6 +12,6 @@ search = __version__ = '{current_version}'
 replace = __version__ = '{new_version}'
 
 [flake8]
-extend-ignore = F401,W504
+extend-ignore = F401,W504,E203
 exclude = __init__.py,*.pyc,tests.py
 paths = ./holidays/ ./tests/