Restore cbook.report_memory, which was deleted in d063dee.

[efiring]

There was no deprecation process, and having something like this
available as an import is very valuable to users suspecting a
memory leak, or simply needing to track memory usage.

[efiring]

As an example of why we need this, or a good replacement, see #6940.

[tacaswell]

It looks like the loop from https://github.com/matplotlib/matplotlib/pull/5360/files#diff-84d254616071e10903ac389d05d109deR36 should be exposed as a cbook function as well?

[tacaswell]

re-milestoned as 2.1 because this has only been removed on master (https://github.com/matplotlib/matplotlib/blob/v2.x/lib/matplotlib/cbook.py#L1439)

[efiring]

@tacaswell, yes, it would be nice to have the whole run_memleak_test() function available as an import, but it relies on a python 3.4+ import, and psutil. We could put it in cbook within a try-except block beginning with the tracemalloc and psutil imports.

[tacaswell]

Merging this as-is as the alternative is much more work than I anticipated due to py3 only imports.