pybind/mgr/tox.ini: add commas in "modules" variable

pybind/mgr/tox.ini: add commas in "modules" variable

since tox v4.0.13, it parses the variables differently, so the newlines
in a variable are passed right to the command referencing it. so we now
have failure like:
```
flake8: commands[0] /home/jenkins-build/build/workspace/ceph-pull-requests/src/pybind/mgr> flake8 --config=tox.ini alerts
flake8: commands[1] /home/jenkins-build/build/workspace/ceph-pull-requests/src/pybind/mgr> balancer
flake8: exit 2 (0.00 seconds) /home/jenkins-build/build/workspace/ceph-pull-requests/src/pybind/mgr> balancer
flake8: FAIL ✖ in 3.33 seconds
```
so we have to add comma as line continuation separator to address
this problem.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>