feat: Add pre-commit checks

* Base pre-commit checks for trailing whitespces, valid files, etc
* pyupgrade with PY37+ syntax
* Aditya's black fork for codebases with tabs
* Flake8 with Frappe's config

.flake8

@@ -0,0 +1,36 @@
+[flake8]
+ignore =
+    E121,
+    E126,
+    E127,
+    E128,
+    E203,
+    E225,
+    E226,
+    E231,
+    E241,
+    E251,
+    E261,
+    E265,
+    E302,
+    E303,
+    E305,
+    E402,
+    E501,
+    E741,
+    W291,
+    W292,
+    W293,
+    W391,
+    W503,
+    W504,
+    F403,
+    B007,
+    B950,
+    W191,
+    E124, # closing bracket, irritating while writing QB code
+    E131, # continuation line unaligned for hanging indent
+    E123, # closing bracket does not match indentation of opening bracket's line
+    E101, # ensured by use of black
+
+max-line-length = 200

.pre-commit-config.yaml

@@ -0,0 +1,37 @@
+exclude: '.git'
+default_stages: [commit]
+fail_fast: false
+
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.3.0
+    hooks:
+      - id: trailing-whitespace
+        files: "frappe.*"
+        exclude: ".*json$|.*txt$|.*csv|.*md|.*svg"
+      - id: check-yaml
+      - id: check-merge-conflict
+      - id: check-ast
+      - id: check-json
+      - id: check-toml
+      - id: check-yaml
+      - id: debug-statements
+
+  - repo: https://github.com/asottile/pyupgrade
+    rev: v2.34.0
+    hooks:
+      - id: pyupgrade
+        args: ['--py37-plus']
+
+  - repo: https://github.com/adityahase/black
+    rev: 9cb0a69f4d0030cdf687eddf314468b39ed54119
+    hooks:
+      - id: black
+        additional_dependencies: ['click==8.0.4']
+
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.9.2
+    hooks:
+      - id: flake8
+        additional_dependencies: ['flake8-bugbear',]
+        args: ['--config', '.flake8']