Make type checking a seperate tox environment

pypy fails on mypy.stubtest with "ModuleNotFoundError: No module named
'_symtable'". The simplest way to to not run this on pypy is to split
out type checking into a seperate environment and only run it if not
pypy.

In doing so, the encoding warning need not be turned on for the type
checking environment so python does not need to be run in "ignore
warnings" mode to execute mypy.

.github/workflows/main.yml

@@ -65,6 +65,9 @@ jobs:
         run: python -m pip install tox
       - name: Run
         run: tox
+      - name: Run type checking
+        if: matrix.python != 'pypy3.10'
+        run: tox -e type
 
   collateral:
     strategy:

tox.ini

@@ -1,19 +1,16 @@
 [testenv]
-description = perform primary checks (tests, style, types, coverage)
+description = perform primary checks (tests, style, coverage)
 deps =
 setenv =
 	PYTHONWARNDEFAULTENCODING = 1
 commands =
 	pytest {posargs}
-	python -W ignore -m mypy path
-	python -W ignore -m mypy.stubtest path
 usedevelop = True
 extras =
 	test
 	check
 	cover
 	enabler
-	type
 
 [testenv:diffcov]
 description = run tests and check that diff from main is covered
@@ -35,6 +32,15 @@ commands =
 	python -m sphinx -W --keep-going . {toxinidir}/build/html
 	python -m sphinxlint
 
+[testenv:type]
+description = perform type checking
+setenv =
+extras =
+	type
+commands =
+	python -m mypy path
+	python -m mypy.stubtest path
+
 [testenv:finalize]
 description = assemble changelog and tag a release
 skip_install = True