Fix broken link to proxy artists documentation

[deepyaman]

http://matplotlib.org/users/legend_guide.html#using-proxy-artist refers
to a named anchor that is unavailable. Use
http://matplotlib.org/users/legend_guide.html#creating-artists-specifically-for-adding-to-the-legend-aka-proxy-artists
instead.

[dstansby]

Thanks for opening a PR! We require any new code to require to the PEP8 guidelines - it looks like one of your added lines is too long (see here for the error https://travis-ci.org/matplotlib/matplotlib/jobs/286021773#L2930) - could you make it less than 80 characters long?

[deepyaman]

@dstansby Thank you for your review. I was aware that it violates the 80-character rule of PEP8, but my understanding was that long URLs are an exception (for copying, readability, etc.). See https://google.github.io/styleguide/pyguide.html#Line_length, https://stackoverflow.com/questions/10739843/how-should-i-format-a-long-url-in-a-python-comment-and-still-be-pep8-compliant, or even from the matplotlib repo,

matplotlib/examples/user_interfaces/embedding_in_qt4_wtoolbar_sgskip.py

Line 61 in 7897b85

# http://matplotlib.org/users/navigation_toolbar.html#navigation-keyboard-shortcuts

I'd even prefer to put the whole URL on one line, but I moved the fragment to be consistent with the current version and more in line with the PEP8 recommendation.

That being said, if I should still make it less than 80 characters long, I can change it to the following:

            if handler is None:
                warnings.warn(
                    "Legend does not support {!r} instances.\nA proxy artist "
                    "may be used instead.\nSee: "
                    "http://matplotlib.org/users/legend_guide.html"
                    "#creating-artists-specifically-for-adding-to-the-legend-"
                    "aka-proxy-artists".format(orig_handle)
                )

[jklymak]

Does #noqa work for pytest --pep8?

[anntzer]

You can add entries in pytest.ini, but here I'd just split the string tbh (ignore entries in pytest.ini is a bit of a giant hammer).

[tacaswell]

you can also use \ to split a line

That is a bit of an absurd ref target...

[jklymak]

This looks good to me. Link works...

[jklymak]

Thanks @deepyaman !