MAINT: use collections.abc for 3.7

[andyfaff]

scipy is getting some issues when trying to test on Python 3.7 because of a DeprecationWarning:

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

This PR just changes a few instances of collections.X into collections.abc.X.

_____________________ TestDendrogram.test_dendrogram_plot _____________________
[gw1] win32 -- Python 3.7.0 C:\Python37-x64\python.exe
self = <scipy.cluster.tests.test_hierarchy.TestDendrogram object at 0x0000005E63E60160>
    @pytest.mark.skipif(not have_matplotlib, reason="no matplotlib")
    def test_dendrogram_plot(self):
        for orientation in ['top', 'bottom', 'left', 'right']:
>           self.check_dendrogram_plot(orientation)
orientation = 'top'
self       = <scipy.cluster.tests.test_hierarchy.TestDendrogram object at 0x0000005E63E60160>
C:\Python37-x64\lib\site-packages\scipy\cluster\tests\test_hierarchy.py:822: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
C:\Python37-x64\lib\site-packages\scipy\cluster\tests\test_hierarchy.py:842: in check_dendrogram_plot
    ax = fig.add_subplot(221)
C:\Python37-x64\lib\site-packages\matplotlib\figure.py:1239: in add_subplot
    a = subplot_class_factory(projection_class)(self, *args, **kwargs)
C:\Python37-x64\lib\site-packages\matplotlib\axes\_subplots.py:77: in __init__
    self._axes_class.__init__(self, fig, self.figbox, **kwargs)
C:\Python37-x64\lib\site-packages\matplotlib\axes\_base.py:523: in __init__
    self.cla()
C:\Python37-x64\lib\site-packages\matplotlib\axes\_base.py:1032: in cla
    self.set_xlim(0, 1)
C:\Python37-x64\lib\site-packages\matplotlib\axes\_base.py:3110: in set_xlim
    self._process_unit_info(xdata=(left, right))
C:\Python37-x64\lib\site-packages\matplotlib\axes\_base.py:2139: in _process_unit_info
    self.xaxis.update_units(xdata)
C:\Python37-x64\lib\site-packages\matplotlib\axis.py:1463: in update_units
    converter = munits.registry.get_converter(data)
C:\Python37-x64\lib\site-packages\matplotlib\units.py:188: in get_converter
    thisx = safe_first_element(x)
C:\Python37-x64\lib\site-packages\matplotlib\cbook\__init__.py:2347: in safe_first_element
    if isinstance(obj, collections.Iterator):
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
name = 'Iterator'
    def __getattr__(name):
        # For backwards compatibility, continue to make the collections ABCs
        # through Python 3.6 available through the collections module.
        # Note, no new collections ABCs were added in Python 3.7
        if name in _collections_abc.__all__:
            obj = getattr(_collections_abc, name)
            import warnings
            warnings.warn("Using or importing the ABCs from 'collections' instead "
                          "of from 'collections.abc' is deprecated, "
                          "and in 3.8 it will stop working",
>                         DeprecationWarning, stacklevel=2)
E           DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
name       = 'Iterator'
obj        = <class 'collections.abc.Iterator'>
warnings   = <module 'warnings' from 'C:\\Python37-x64\\lib\\warnings.py'>
C:\Python37-x64\lib\collections\__init__.py:52: DeprecationWarning

[timhoffm]

Thanks! After reverting the change to OrderedDict this will be good.

[timhoffm]

OrderedDict is not in abc

[andyfaff]

Lets see if this works.

[anntzer]

need a line break here, the style checker is not happy

[anntzer]

@tacaswell I think avoiding a warning on Py3.7 makes this worth milestoning for mpl3.0?

[anntzer]

OSX failure is irrelevant and will be "fixed" (or worked around, at least) by #11763. Allowing myself to remilestone to 3.0 per above.

[anntzer]

Thanks for the PR.

[tacaswell]

@meeseeksdev backport to v2.2.x

[lumberbot-app]

There seem to be a conflict, please backport manually