Backport PR matplotlib#29834: TST: pin flake8

Author: jklymak

environment.yml

@@ -53,7 +53,7 @@ dependencies:
   # testing
   - black<24
   - coverage
-  - flake8>=3.8
+  - flake8>=3.8,<7.2
   - flake8-docstrings>=1.4.0
   - gtk4
   - ipykernel

requirements/testing/flake8.txt

@@ -1,6 +1,6 @@
 # Extra pip requirements for the GitHub Actions flake8 build
 
-flake8>=3.8
+flake8>=3.8,<7.2
 # versions less than 5.1.0 raise on some interp'd docstrings
 pydocstyle>=5.1.0
 # 1.4.0 adds docstring-convention=all