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This is only necessary if `get_internals` is called for the first time in a given module when the running thread is in a GIL-released state.

Fixes #1364

* Fix warning that not including a cmake source or build dir will be a fatal error (it is now on newest CMakes)
    * Fixes appveyor
* Travis uses CMake 3.9 for more than a year now
* Travis dropped sudo: false in December
* Dropping Sphinx 2
- clang7: Suppress self-assign warnings; fix missing virtual dtors
- pypy:
  - Keep old version (newer stuff breaks)
  - Pin packages to extra index for speed
- travis:
  - Make docker explicit; remove docker if not needed
  - Make commands more verbose (for debugging / repro)
  - Make Ubuntu dist explicit per job
- Fix Windows
- Add names to travis

This avoids GIL deadlocking when pybind11 tries to acquire the GIL in a thread that already acquired it using standard Python API (e.g. when running from a Python thread).