Overload '<' and '>' operators in `LazyUUIDTaskSet`

[RonMcKay]

In #96 the request was to overload the '<' and '>' operators in LazyUUIDTaskSet. I did that in this PR by adding the methods isstrictsubset and isstrictsuperset to the class. If I understood the issue correct this was everything that needs to be implemented for that. If I missed something I am happy for a remark.

[RonMcKay]

I added some commits to address the discussion above. In summary the following changed:

• Implemented the __lt__ and __gt__ methods by using the comparison operators of set
• Refactored the issubset and issuperset methods by also using the comparison operators of set
• Added unittests for __lt__, __gt__, __le__ and __ge__. Please let me know if I have forgotten a corner case.
• Fixed a wrong indentation in tests.py

All tests are passing for me.

Edit: Sorry for the force push but my IDE seems to have introduced some unwanted changes. Fixed that now.