Fix warning of using is not for string compare

[digizeph]

Fix the following warning in python3.8

/usr/local/lib/python3.8/site-packages/netaddr-0.7.19-py3.8.egg/netaddr/strategy/__init__.py:189: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if word_sep is not '':

[sbaerlocher]

Would be cool if the fix is implemented and a new version is created.

[max-wittig]

Ansible seems to have upgraded their version now. Now it shows up everywhere.

@snordhausen
@drkjam

[sbaerlocher]

As I have seen, it would be fixed in the master branch already but a new release has been made so far!

@snordhausen @drkjam Can one of you make a new release with the fix?

[ssbarnea]

Well... another fix for the same old problem.

[digizeph]

Well... another fix for the same old problem.

(Well, your tone seems to suggest this was a duplicate PR. No other PR I can find fixes this when I created this PR.)

[sbaerlocher]

@digizeph It is fixed in the master branch but there is no new release where it is fixed.

https://github.com/netaddr/netaddr/blob/master/netaddr/strategy/__init__.py#L189

[digizeph]

Ah, got it. Thanks, @sbaerlocher!

[jstasiak]

Thanks for this, another patch addressing this has already been merged into the master branch (now released in 0.7.20), sorry about the confusion about the branches and the default branch, this should also be addressed now.