Remove flake8 E303 (too many blank lines) (#20225)

Summary:
similar to too few blank lines, I feel like this is not important enough to warrant breaking signal for all linters when it's violated.
Pull Request resolved: #20225

Differential Revision: D15243480

Pulled By: suo

fbshipit-source-id: 37cdc18daf09e07081e42b69c72d331d81660217

.flake8

@@ -4,7 +4,7 @@ max-line-length = 120
 # C408 ignored because we like the dict keyword argument syntax
 # E501 is not flexible enough, we're using B950 instead
 ignore =
-    E203,E305,E402,E501,E721,E741,F403,F405,F821,F841,F999,W503,W504,C408,E302,W291,
+    E203,E305,E402,E501,E721,E741,F403,F405,F821,F841,F999,W503,W504,C408,E302,W291,E303,
     # these ignores are from flake8-bugbear; please fix!
     B007,B008,
     # these ignores are from flake8-comprehensions; please fix!