Segfaults on tests for Python 3.7.0

[SethMMorton]

Inconsistently, and at "random" points during testing, I experience segmentation faults on Python 3.7.0. It happens no matter the OS (Mac, Windows, Linux). I strongly believe that this is not a bug in fastnumbers but rather with Python itself. My logic for this conclusion is

• There is an open issue on bugs.python.org (https://bugs.python.org/issue34087, called "int(s), float(s) and others may cause segmentation fault") that points out that a change to the unicode handling internal function _PyUnicode_TransformDecimalAndSpaceToASCII that is called by int and float can introduce segfaults.
• There is no difference in code within fastnumbers between Python 3.6 and 3.7, and no segfaults occur on Python 3.6.

I will leave this issue open till Python 3.7.1 is released (which has a patch for the segfault) and I can verify that this resolves the issue.

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In interesting coincidence - it is the fact that _PyUnicode_TransformDecimalAndSpaceToASCII changed in 3.7 that started PR #16, but it was because its behavior changed, not because of segfaults.

[SethMMorton]

When appveyor/ci#2735 is merged, I will be able to validate and (hopefully) close this issue.

[SethMMorton]

Validated by builds https://travis-ci.org/SethMMorton/fastnumbers/builds/499655055 and https://ci.appveyor.com/project/SethMMorton/fastnumbers/builds/22710401 passing on the first attempt.