filters: do not use imp in ImportableModuleFilter
filters: do not use imp in ImportableModuleFilter

The imp module is deprecated, so replace it with calls to importlib.

Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

sphinx-contrib/spelling,sphinx-contrib/spelling