Fix python syntax to suppress 3.8 warnings about "is with a literal"

[aptalca]

Python 3.8 is now displaying a warning when is is used with a literal.

Here are the relevant errors with 2.4.0:

/usr/lib/python3.8/site-packages/CloudFlare/cloudflare.py:839: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if email is '':
/usr/lib/python3.8/site-packages/CloudFlare/cloudflare.py:841: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if token is '':
/usr/lib/python3.8/site-packages/CloudFlare/cloudflare.py:843: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if certtoken is '':

See python/cpython#9642 for more info

closes #83

[mahtin]

Yeah - I found a bunch of these and pushed it as branch fix-python-3.8 which is merged into master now and released as 2.4.1 on https://pypi.org/project/cloudflare/ (i.e. pip3 install cloudflare)