Fix flake8 config (#30907)

* Fix flake8 config

* Add excludes for flake8

.flake8

@@ -1,5 +1,24 @@
 [flake8]
 ignore =
-    W291,  # trailing whitespace; the standard tidy process will enforce no trailing whitespace
+    # trailing whitespace; the standard tidy process will enforce no trailing whitespace
-    W503,  # linebreak before binary operator; replaced by W504 - linebreak after binary operator
+    W291,
-    E501,  # 80 character line length; the standard tidy process will enforce line length
+    # linebreak before binary operator; replaced by W504 - linebreak after binary operator
+    W503,
+    # 80 character line length; the standard tidy process will enforce line length
+    E501
+exclude =
+    # temporary local files
+    target
+    __pycache__
+    python/_venv*
+    # upstream
+    third_party
+    python/mach
+    components
+    tests
+    # We should fix those
+    etc/ci
+    ./python/wpt/run.py
+    ./python/tidy/tidy.py
+    ./etc/servo_automation_screenshot.py
+    ./etc/memory_reports_over_time.py