* fix: respect PYTHON_VERSION if set in classic mode

* fix: add warning when using PYTHON_VERSION

tests: fix CI by including <algorithm> to stop MSVC from complaining about std::count_if in tests/test_sequences_and_iterators.cpp (#2435)

Throw exception on returning a unique_ptr or shared_ptr nullptr (or any other holder type) from py::init, rather than crashing (#2430)

* Add note that VS2017 requires /permissive- to build in C++17 mode

* ci: test C++17 on MSVC 2017

* ci: args1/2, use args to override max cxx

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

Adapt code example in advanced/classes.rst to new handling of forgetting to call the superclass __init__ (#2429)

The main change is to treat error_already_set as a separate category
of exception that arises in different circumstances and needs to be
handled differently. The asymmetry between Python and C++ exceptions
is further emphasized.

* fix: reduce target collision in add_submodule mode

Closes #2420

* fix: update CMakeLists.txt

CPython configured with `--with-pydebug` could not use `pybind11_add_module`

* docs: move CONTRIBUTING

* docs: clarify PyPy promise

* tests: keep source dir clean

* ci: make first build inplace

* ci: drop dev setting (wasn't doing anything)

* tests: warn if source directory is dirty

* feat: FindPython support

* refactor: rename to PYBIND11_FINDPYTHON

* docs: Caps fixes

* feat: NOPYTHON mode

* test: check simple call

* docs: add changelog/upgrade guide

* feat: Support Python3 and Python2

* refactor: Use targets in tests

* fix: support CMake 3.4+

* feat: classic search also finds virtual environments

* docs: some updates from @wjakob's review

* fix: wrong name for QUIET mode variable, reported by @skoslowski

* refactor: cleaner output messaging

* fix: support debug Python's in FindPython mode too

* fixup! refactor: cleaner output messaging

* fix: missing pybind11_FOUND and pybind11_INCLUDE_DIR restored to subdir mode

* fix: nicer reporting of Python / PyPy

* fix: out-of-order variable fix

* docs: minor last-minute cleanup

* fix: throwing repr caused a segfault

* fixup! ci: include Python 3.9 RC1 (#2387)

* 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`.