tox: split formatting testenv from flake8

Split the job of the formatting testenv that was previously running
both flake8 and black. Let formatting focus on code formatting only
(using black) and add a new testenv "flake8" for running said tool.
This avoids overloading the testenvs and allows for more flexibility
like running these envs with tox's --parallel option.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

Author: phlogistonjohn

tox.ini

@@ -1,6 +1,6 @@
 
 [tox]
-envlist = formatting, {py3,py39}-mypy, py3, py39, schemacheck, py3-sys
+envlist = flake8, formatting, {py3,py39}-mypy, py3, py39, schemacheck, py3-sys
 isolated_build = True
 
 [testenv]
@@ -51,14 +51,19 @@ allowlist_externals =
     /usr/bin/py.test
 
 [testenv:formatting]
-description = Check the formatting for the source files
+description = Check the style/formatting for the source files
 deps =
-    flake8
     black>=24, <25
 commands =
-    flake8 sambacc tests
     black --check -v .
 
+[testenv:flake8]
+description = Basic python linting for the source files
+deps =
+    flake8
+commands =
+    flake8 sambacc tests
+
 [testenv:schemacheck]
 description = Check the JSON Schema files are valid
 deps =