tox: replace flake8 with a full pre-commit.

This runs much more.
Also, the plain `flake8` complained, because it was configured differently:

```
$ toxuv -e flake8
flake8: commands[0]> flake8 --ignore E501 setup.py src
src/mr/developer/commands.py:710:50: E203 whitespace before ':'
src/mr/developer/extension.py:245:63: E203 whitespace before ':'
src/mr/developer/svn.py:214:17: W503 line break before binary operator
flake8: exit 1 (0.29 seconds) /Users/maurits/community/mr.developer> flake8 --ignore E501 setup.py src
```

Author: mauritsvanrees

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py310,py311,py312,py313,py314,py315,flake8
+envlist = lint,py310,py311,py312,py313,py314,py315
 
 [base]
 deps =
@@ -15,10 +15,13 @@ deps =
     configparser: configparser
 
 
-[testenv:flake8]
-commands = flake8 --ignore E501 setup.py src
-deps = flake8
+[testenv:lint]
+description = run linters that will help improve the code style
 skip_install = true
+deps =
+    pre-commit
+commands =
+    pre-commit run -a
 
 
 [pytest]