DEV: use flake8 < 6.0 in environment.yml

6.0 removed the `--diff` argument that we need. CI is pinned to
version 3.9.2, but any version below 6 seems to work well.

[ci skip]

Author: rgommers

dev.py

@@ -889,7 +889,7 @@ def task_unicode_check():
         'basename': 'unicode-check',
         'actions': [str(Dirs().root / 'tools' / 'unicode-check.py')],
         'doc': 'Check for disallowed Unicode characters in the SciPy Python '
-                'and Cython source code.',
+               'and Cython source code.',
     }
 
 

environment.yml

@@ -39,7 +39,7 @@ dependencies:
   - pydata-sphinx-theme==0.9.0
   - sphinx-design
   # For linting
-  - flake8
+  - flake8 < 6.0
   # Some optional test dependencies
   - mpmath
   - gmpy2