Corrects import of MutableMapping class for python 3.3+

MutableMapping class is now imported from collections.abc instead
of collections. This change was made to python standard library in
version 3.3. collections.MutableMapping has been provided for
backwards compatability up to python 3.7 but will no longer be
available from python 3.8.

A try/catch statement is used with the import to retain backwords
compatibility with python version < 3.3 including python 2.

Author: jathpala

nibabel/streamlines/tractogram.py

@@ -1,9 +1,14 @@
 import copy
 import numbers
 import numpy as np
-import collections
 from warnings import warn
 
+try:
+    from collections.abc import MutableMapping
+except ImportError:
+    # PY2 compatibility
+    from collections import MutableMapping
+
 from nibabel.affines import apply_affine
 
 from .array_sequence import ArraySequence
@@ -19,7 +24,7 @@ def is_lazy_dict(obj):
     return is_data_dict(obj) and callable(list(obj.store.values())[0])
 
 
-class SliceableDataDict(collections.MutableMapping):
+class SliceableDataDict(MutableMapping):
     """ Dictionary for which key access can do slicing on the values.
 
     This container behaves like a standard dictionary but extends key access to
@@ -181,7 +186,7 @@ def _extend_entry(self, key, value):
         self[key].extend(value)
 
 
-class LazyDict(collections.MutableMapping):
+class LazyDict(MutableMapping):
     """ Dictionary of generator functions.
 
     This container behaves like a dictionary but it makes sure its elements are