pybind/mgr: fix mypy env to allow use with >1 python version

tox supports automatically running tools with different python versions
if you name your envs properly. Continue to permit the use of `mypy` as
an env (as the default) but also allow a selection of python
versions like `py38-mypy`.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

src/pybind/mgr/tox.ini

@@ -65,12 +65,11 @@ commands =
     pytest {posargs:cephadm/tests/test_ssh.py}
 
 
-[testenv:mypy]
+[testenv:{,py37-,py38-,py39-,py310-}mypy]
 setenv =
     MYPYPATH = {toxinidir}/..
 passenv =
     MYPYPATH
-basepython = python3
 deps =
     -rrequirements.txt
     -c{toxinidir}/../../mypy-constrains.txt