move development dependencies to a requirements file

This makes it easier to update the pinned versions using pip freeze
output.

Author: dhellmann

requirements-dev.txt

@@ -0,0 +1,7 @@
+astpretty==2.1.0
+flake8==7.1.1
+flake8-debug==0.2.0
+isort==5.13.2
+mccabe==0.7.0
+pycodestyle==2.12.1
+pyflakes==3.2.0

tox.ini

@@ -34,17 +34,13 @@ commands=
       --log-level DEBUG
 
 [testenv:style]
-deps =
-    isort>=5.0.1
+deps = -r requirements-dev.txt
 commands =
     isort .
 skip_install = true
 
 [testenv:linter]
-deps=
-    flake8==4.0.1
-    flake8-debug==0.1.0
-    isort>=5.0.1
+deps = -r requirements-dev.txt
 setenv =
     BUILD=linter
 commands =