bitstring-3.1.6

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July 9th 2019: version 3.1.6 released

A long overdue maintenace release with some fixes.

• Fixed immutability bug. Bug 176.
• Fixed failure of __contains__ in some circumstances. Bug 180.
• Better handling of open files. Bug 186.
• Better Python 2/3 check.
• Making unit tests easier to run.
• Allowing length of 1 to be specified for bools. (Thanks to LemonPi)