Revert default parser, add FutureWarning for v0.4.0 change

[tlambert03]

I'd like to release a new version with the awesome lxml parser added by @Nicholas-Schaub, but I'm just a bit wary of changing up the default all in one go (since I can't be certain that there aren't some consequences for untested applications).

This PR sets the default parser to None. If parser is None, it shows a FutureWarning, explaining that the behavior will change in v0.4.0, and then defaults back to the xmlschema parser. To silence the warning you can either opt in to the new behavior with parser='lxml', or retain the old behavior with parser='xmlschema'

@jmuhlich @Nicholas-Schaub, lemme know if this is ok with you guys. If so, I'll merge and release 0.3.0, then will release 0.4.0 with the new default (and no more warning in a month or two)

[Nicholas-Schaub]

Works for me.

[tlambert03]

Since we discussed it in person, I'm going to optimistically assume that @jmuhlich is on board with this and merge/release 😂