Add compatibility for AST changes in Python 3.7

[mingwandroid]

Not sure who to ping on this trying @mwiebe, @cpcloud, @dhirschfeld, sorry for the spam if you no longer work on datashape.

[dhirschfeld]

@llllllllll might be interested. I'll try and take a look when I find the time...

Two comments though:

• I suppose I could test by installing a py37 pre-release binary for Windows but it's quite a hurdle as compared to conda create -n test-py37 python=3.7. Is there any interest at Anaconda in making python pre-releases available (on a dev-channel)?
• blaze/datashape/odo are barely limping along as it is. I think they were great projects but I'm not sure I see a future for them without some corporate backing. Given the emergence of ndtypes, what future is there for datashape?

...that said, thanks for the pull request - if there is a future for datashape it will be needed!

[mingwandroid]

With my PR I see a later test failure, not sure if my PR causes this or not though:

=================================== FAILURES ===================================
________________________________ test_validate _________________________________

    def test_validate():
        assert validate(int, 1)
>       assert validate('int', 1)
E       AssertionError: assert False
E        +  where False = validate('int', 1)

I suppose I could test by installing a py37 pre-release binary for Windows but it's quite a hurdle as compared to conda create -n test-py37 python=3.7. Is there any interest at Anaconda in making python pre-releases available (on a dev-channel)?

I am in the process of seeing what state Python 3.7 is in with respect to the packages we need to build for the Anaconda Distribution. This PR is a part of that. I haven't tackled Windows yet apart from trying to build Python 3.7.0b3 and failing and moving back to Linux and macOS instead. I need to figure out why Windows failed though. My current plan is to release as full a suite of test packages as a I can on the c3i_test channel over the next few days. Whether that includes Windows or not remains to be seen.

[mingwandroid]

The test failure is down to using NumPy 1.14 in the tests.