Fix HTML handling of </>

[facelessuser]

Fixes #1528

[mschoettle]

@facelessuser Sorry for the late feedback. I just now updated our lock file which upgraded markdown which includes this fix and started getting build failures in mkdocs. It happens in a page that contains "</>". I tracked it down to this change. In particular, the change to htmlparser.starttagopen:

https://github.com/facelessuser/markdown/blob/4c0c60575eea3d9795a3a8db7c8f056da9511083/markdown/htmlparser.py#L46

mkdocs doesn't seem to use HTMLExtractor but the change to htmlparser.starttagopen does affect it.

I can raise a new issue if you think this is worth tracking.

[facelessuser]

@mschoettle This issue is fixed in markdown, but mkdocs derives a class from our HTML parser. I submitted a PR to fix this issue, but they have refused to do anything with it.
mkdocs/mkdocs#4007