Add support for Python 3.7

[asherf]

Second attempt... bumping some reqs this time

[coveralls]

Coverage increased (+0.6%) to 98.83% when pulling 978321b on asherf:py37 into 907240b on vrcmarcos:master.

[asherf]

@vrcmarcos seems like an old version of pytest was used, newer versions of pytest removed deprecated python APIs that no longer work in python 3.7.

[vrcmarcos]

Thank you!