Add black config to pyproject.toml (#3493)

* chore: add black config to pyproject.toml

* enhance: make files will now use settings from pyproject.toml

* docs: update CONTRIBUTING.md

pyproject.toml

@@ -1,3 +1,22 @@
+[tool.black]
+    line-length = 99
+    target-version = ['py38']
+    include = '\.py$'
+    exclude = '''
+    /(
+          \.eggs
+        | \.git
+        | \.hg
+        | \.mypy_cache
+        | \.tox
+        | \.venv
+        | _build
+        | buck-out
+        | build
+        | dist
+    )/
+    '''
+
 [tool.towncrier]
     package = "redbot"
     filename = "CHANGELOG.rst"