MAINT: Remove LooseVersion

[larsoner]

I don't like seeing in my Python 3.10 builds:

/home/larsoner/python/sphinx-gallery/sphinx_gallery/py_source_parser.py:105: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.           

So I removed one unnecessary comparison about python version (we require 3.7 already), and another involving matplotlib version that just triaged which GIF writer to use -- 3.3.1 is almost 2 years old, and the GIF-writing bug will only happen if someone actually uses GIFs in their docs and uses a quite old matplotlib version, which I think is rare enough not to worry about.

[larsoner]

Pushed a fix for this, too:

  File "/home/circleci/python_env/lib/python3.8/site-packages/sphinx_gallery/backreferences.py", line 99, in get_mapping
    obj = getattr(obj, level)
RuntimeError: wrapped C/C++ object of type PyQtGraphBrowser has been deleted

We just need to except Exception rather than except AttributeError