pybind/mgr/dashboard: bump flake8 to 3.9.0

pybind/mgr/dashboard: bump flake8 to 3.9.0

to address the failure of

ERROR: Cannot install -r requirements-lint.txt (line 2) and -r requirements-lint.txt (line 8) because these package versions have conflicting dependencies.

The conflict is caused by:
    flake8 3.8.4 depends on pycodestyle<2.7.0 and >=2.6.0a1
    autopep8 1.5.6 depends on pycodestyle>=2.7.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

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