pybind/mgr/dashboard/.pylintrc: silence more pylint warnings

pybind/mgr/dashboard/.pylintrc: silence more pylint warnings

this change is not cherry-picked from master, as we don't have following
warnings in master, like:

************* Module dashboard.controllers.saml2
        intern-builtin,
controllers/saml2.py:57:8: R1720: Unnecessary "else" after "raise" (no-else-raise)

also bump up the versions of pylint and astroid, so they
can work with python3.8.

see
https://github.com/PyCQA/astroid/commit/28fc86f260f0cd9433d0603da81ee52045f9e4c3

Signed-off-by: Kefu Chai <kchai@redhat.com>