Importing ABC directly from collections will be removed in Python 3.10

[tirkarthi]

Use collections.abc instead

texar/tf/modules/encoders/hierarchical_encoders.py
259:            if not isinstance(medium, collections.Sequence):
358:        if isinstance(x, collections.Sequence):

texar/tf/utils/utils.py
580:        if isinstance(value, collections.MutableMapping):