Minor tox.ini improvement: use base = mypy

Declaring a `base` environment allows you to declare a parallel
environment to an existing one with some settings overridden. This
lets us avoid the generative environment listing syntax (slightly
obscure as a feature, not everyone knows about it) and having two
`commands` lists fully differentiated by a factor.

The config here is 100% equivalent to what we had before, but is
easier to read and understand for developers less immersed in tox.

tox.ini

@@ -42,13 +42,13 @@ deps = pre-commit
 skip_install = true
 commands = pre-commit run --all-files
 
-[testenv:mypy{,-test}]
-commands_pre =
-deps =
-    -r requirements/py{py_dot_ver}/typing.txt
-commands =
-    !test: mypy src/ {posargs}
-    test: mypy --show-error-codes --warn-unused-ignores tests/non-pytest/mypy-ignore-tests/
+[testenv:mypy]
+deps = -r requirements/py{py_dot_ver}/typing.txt
+commands = mypy src/ {posargs}
+
+[testenv:mypy-test]
+base = mypy
+commands = mypy --show-error-codes --warn-unused-ignores tests/non-pytest/mypy-ignore-tests/
 
 [testenv:test-lazy-imports]
 deps = -r requirements/py{py_dot_ver}/test.txt