Modified Vector STL bind initialization from a buffer type with optimization for simple arrays (#2298)

* Modified Vector STL bind initialization from a buffer type with optimization for simple arrays

* Add subtests to demonstrate processing Python buffer protocol objects with step > 1

* Fixed memoryview step test to only run on Python 3+

* Modified Vector constructor from buffer to return by value for readability

These tests will also alert us to any behavior changes across Python and PyPy versions.

Hardening tests in preparation for changing `pybind11::str` to only hold `PyUnicodeObject` (NOT also `bytes`). Note that this test exposes that `pybind11::str` can also hold `bytes`.

* Enable py::ellipsis on Python 2

* Enable py::ellipsis tests on Python 2 and mention `Ellipsis` in the docs

Added a check on CMake 3.12+ to make sure the file listing is up to date. And caught a missing file with it!

Fixes issue #1878

* add uninstall target for cmake

* only add target when built as master project

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

fix: include PYTHON_IS_DEBUG

fixes #2191