Drop Python 2.6 support and add 3.7
PIP requires Python 2.7, so it is not possible to test using anything prior to that.
Python 3.7 has been out for a while, so we can test against it.
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PIP requires Python 2.7, so it is not possible to test using anything prior to that.
Python 3.7 has been out for a while, so we can test against it.