/dom_helpers, parser/ fix trivial style warnings

[pawelmhm]

that appear in Python 3.8.2

I get following warnings in 3.8

/home/pawel/.pyenv/versions/3.8.2/lib/python3.8/site-packages/mf2py/parser.py:409: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if x is not "":
/home/pawel/.pyenv/versions/3.8.2/lib/python3.8/site-packages/mf2py/dom_helpers.py:144: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  results = [t for t in text_collection(el, replace_img, img_to_src, base_url) if t is not '']

[tommorris]

This looks fine to me, and once Travis has finished being extremely slow, I'll happily merge it in.

Context for the change that I may as well document here: Python 3.8 is now emitting SyntaxWarning errors where is or is not is used with numeric or string types. This is rather an opinionated change made in bpo-34850. I guess whether we like this change, that's what Python has decided, so it is better to fix it than not.

Also, this PR has prompted me to add issues #159 and #160 because of Travis being sloooow.

[tommorris]

Merged, thanks again @pawelmhm.