Compatibility with PyMongo 3.7

[IlyaSkriblovsky]

Fixing #232

Copy-pasted some internal pymongo utility methods into txmongo so future pymongo updates won't break txmongo.

[IlyaSkriblovsky]

I've also noticed that some of our tests are failing with MongoDB 3.6. Will look into it too.

[coveralls]

Coverage decreased (-0.8%) to 94.373% when pulling ccdf224 on IlyaSkriblovsky:pymongo-3.7-compatibility into 2074262 on twisted:master.

[coveralls]

Coverage decreased (-0.9%) to 94.268% when pulling cf323c9 on IlyaSkriblovsky:pymongo-3.7-compatibility into 2074262 on twisted:master.

[coveralls]

Coverage decreased (-0.9%) to 94.268% when pulling cf323c9 on IlyaSkriblovsky:pymongo-3.7-compatibility into 2074262 on twisted:master.

[psi29a]

Should we start keeping documentation stating where we lifted code from PyMongo? I can imagine that in the future that something breaks that was 'fixed' in a later version of PyMongo.

In comment:

• reason why we copied with link to issue
• which pymongo version, maybe even link to the tagged released file/line

What do you think?

[IlyaSkriblovsky]

I think this is great idea. Going to add this, thanks.

[IlyaSkriblovsky]

@psi29a I've added references to original PyMongo sources we have copied code from. I'm not sure how to specify "reason why we copied" because in all cases it will be "because we need to do the same thing PyMongo does and we don't want to be at risk when PyMongo will change its private functions". All that we have copied are some useful utility functions that deal with MongoDB responses and convert them to python objects or raise exceptions.

[psi29a]

That's good enough, that we know it's copied can give us a clue.

[IlyaSkriblovsky]

If this looks good I think we should make a release because currently pip install txmongo is installing non-working environment.