Use `Mapping` from `collections.abc` instead of `collections`

[Achilles1515]

Running the library from the latest version of Python gives the following deprecation warning:

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working
  class DWFile(collections.Mapping):

So use Mapping from the abc module instead.

[costerwi]

This breaks compatibility with Python <3.3. I wonder if there's a way to catch and fall back to the old collections?

[Achilles1515]

@costerwi Fixed.

[costerwi]

Thanks