Backport PR #41462: CI: Fix changed flake8 error message after upgrade (#41462)

.pre-commit-config.yaml

@@ -4,7 +4,7 @@ repos:
     hooks:
     -   id: black
 -   repo: https://gitlab.com/pycqa/flake8
-    rev: 3.8.4
+    rev: 3.9.2
     hooks:
     -   id: flake8
         additional_dependencies: [flake8-comprehensions>=3.1.0]

environment.yml

@@ -20,7 +20,7 @@ dependencies:
   # code checks
   - black=20.8b1
   - cpplint
-  - flake8
+  - flake8=3.9.2
   - flake8-comprehensions>=3.1.0  # used by flake8, linting of unnecessary comprehensions
   - isort>=5.2.1  # check that imports are in the right order
   - mypy=0.782

requirements-dev.txt

@@ -8,7 +8,7 @@ asv
 cython>=0.29.21
 black==20.8b1
 cpplint
-flake8
+flake8==3.9.2
 flake8-comprehensions>=3.1.0
 isort>=5.2.1
 mypy==0.782

scripts/tests/test_validate_docstrings.py

@@ -165,7 +165,7 @@ def test_bad_class(self, capsys):
                 "indentation_is_not_a_multiple_of_four",
                 # with flake8 3.9.0, the message ends with four spaces,
                 #  whereas in earlier versions, it ended with "four"
-                ("flake8 error: E111 indentation is not a multiple of     ",),
+                ("flake8 error: E111 indentation is not a multiple of 4",),
             ),
             (
                 "BadDocstrings",