Backend: Add pre-commit hook with black and flake8 along with code formatting(Cloud-CV#2223)

Author: RishabhJain2018

.flake8

@@ -0,0 +1,5 @@
+[flake8]
+ignore = E203, E266, E501, W503, F403, F401, F405, C901, F601
+max-line-length = 79
+max-complexity = 18
+select = B,C,E,F,W,T4,B9

.pre-commit-config.yaml

@@ -0,0 +1,10 @@
+repos:
+-   repo: https://github.com/ambv/black
+    rev: stable
+    hooks:
+    - id: black
+      language_version: python3.7
+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v1.2.3
+    hooks:
+    - id: flake8