Fix collections.abc import for Python 3.10

[hugovk]

Since Python 3.3, MutableMapping has been available in collections.abc.

Since Python 3.9, MutableMapping is only available in collections.abc.

Python 3.9.0a2+ (heads/master:ec007cb, Jan  5 2020, 12:34:22)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import collections
>>> collections.MutableMapping
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'collections' has no attribute 'MutableMapping'
>>> collections.abc.MutableMapping
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'collections' has no attribute 'abc'
>>> import collections.abc
>>> collections.abc.MutableMapping
<class 'collections.abc.MutableMapping'>
>>>

Python 3.9 is in alpha with full release due in October.

Fedora are already making early builds for 3.9 and found this problem: https://bugzilla.redhat.com/show_bug.cgi?id=1792029.

[hugovk]

Updated title because they postponed this breaking change to Python 3.10 (due out October 2021):

Python 3.9.1 (v3.9.1:1e5d33e9b9, Dec  7 2020, 12:10:52)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import collections
>>> collections.MutableMapping
<stdin>:1: 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 'collections.abc.MutableMapping'>
>>> collections.abc.MutableMapping
<class 'collections.abc.MutableMapping'>
>>>

[hugovk]

Rebased on main.

Python 3.10 is now in beta and has an error. 3.10.0 final is due out on 2021-10-04.

• https://www.python.org/dev/peps/pep-0619/#schedule

Python 3.9

$ python3.9 -c "import collections; collections.MutableMapping"
<string>:1: 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

Python 3.10

$ python3.10 -c "import collections; collections.MutableMapping"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: module 'collections' has no attribute 'MutableMapping'

[behdad]

I don't know if we still support older Python or what. @anthrotype?

[anthrotype]

Sorry I had missed this one. We support Python 3.6 and above

LGTM