move and update flake8 config

.flake8

@@ -0,0 +1,25 @@
+[flake8]
+extend-select =
+    # bugbear
+    B
+    # bugbear opinions
+    B9
+    # implicit str concat
+    ISC
+extend-ignore =
+    # slice notation whitespace, invalid
+    E203
+    # line length, handled by bugbear B950
+    E501
+    # bare except, handled by bugbear B001
+    E722
+    # zip with strict=, requires python >= 3.10
+    B905
+    # string formatting opinion, B028 renamed to B907
+    B028
+    B907
+# up to 88 allowed by bugbear B950
+max-line-length = 80
+per-file-ignores =
+    # __init__ exports names
+    src/flask/__init__.py: F401