[core] Migrate to pyproject.toml (#6737)

.flake8

@@ -0,0 +1,42 @@
+[flake8]
+max-line-length = 120
+# Following 4 for black compatibility
+# E501: line too long
+# W503: Line break occurred before a binary operator
+# E203: Whitespace before ':'
+# D202 No blank lines allowed after function docstring
+
+# TODO fix flake8
+# D100 Missing docstring in public module
+# D101 Missing docstring in public class
+# D102 Missing docstring in public method
+# D103 Missing docstring in public function
+# D104 Missing docstring in public package
+# D105 Missing docstring in magic method
+# D107 Missing docstring in __init__
+# D200 One-line docstring should fit on one line with quotes
+# D205 1 blank line required between summary line and description
+# D209 Multi-line docstring closing quotes should be on a separate line
+# D400 First line should end with a period
+# D401 First line should be in imperative mood
+
+ignore =
+  E501,
+  W503,
+  E203,
+  D202,
+
+  D100,
+  D101,
+  D102,
+  D103,
+  D104,
+  D105,
+  D107,
+  D200,
+  D205,
+  D209,
+  D400,
+  D401,
+
+exclude = api_pb2.py