* docs: move helpers to .github where allowed

* docs: more guidelines in CONTRIBUTING

* chore: update issue templates

* fix: review from @bstaletic

* refactor: a few points from @rwgk

* docs: more touchup, review changes

* tests: refactor and cleanup

* refactor: more consistent

* tests: vendor six

* tests: more xfails, nicer system

* tests: simplify to info

* tests: suggestions from @YannickJadoul and @bstaletic

* tests: restore some pypy tests that now pass

* tests: rename info to env

* tests: strict False/True

* tests: drop explicit strict=True again

* tests: reduce minimum PyTest to 3.1

To deal with exceptions that hit destructors or other noexcept functions.

Includes fixes to support Python 2.7 and extends documentation on
error handling.

@virtuald and @YannickJadoul both contributed to this PR.

Important gain: uniformity & therefore easier cleanup when we drop PY2 support.
Very slight loss: it was nice to have `str is bytes` as a reminder in this specific context.

Fix logic in test_bytes_to_string

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

* Fix warning C26817: Potentially expensive copy of variable 'vh' in range-for loop. Consider making it a const reference (es.71).

* Replace another instance of `for (auto vh : values_and_holders(...))` with `auto vh &` (found by @bstaletic)

Co-authored-by: Michael Goulding <Michael.Goulding@microsoft.com>
Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>

* fix: add missing signature

* fix: add to array_t too

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