Switching deprecated Thread Local Storage (TLS) usage in Python 3.7 to Thread Specific Storage (TSS) (#1454)

* Switching deprecated Thread Local Storage (TLS) usage in Python 3.7 to Thread Specific Storage (TSS)

* Changing Python version from 3.6 to 3.7 for Travis CI, to match brew's version of Python 3

* Introducing PYBIND11_ macros to switch between TLS and TSS API