Collections import compatibility fix for Python versions 3.8+

[ionite34]

In Python 3.7, importing ABCs directly from the collections module shows a
warning and the import is deprecated since Python 3.8+

See:
python/cpython@c66f9f8

Pull request added different compatible import methods per Python version.

Changes for ipapy/ipastring.py

-from collections import MutableSequence
import sys
if sys.version_info[:2] >= (3, 8):
    from collections.abc import MutableSequence
else:
    from collections import MutableSequence

Changes for ipapy/mapper.py

-from collections import MutableMapping
import sys
if sys.version_info[:2] >= (3, 8):
    from collections.abc import MutableMapping
else:
    from collections import MutableMapping