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* `#error BYE_BYE_GOLDEN_SNAKE`

* Removing everything related to 2.7 from ci.yml

* Commenting-out Centos7

* Removing `PYTHON: 27` from .appveyor.yml

* "PY2" removal, mainly from tests. C++ code is not touched.

* Systematic removal of `u` prefix from `u"..."` and `u'...'` literals. Collateral cleanup of a couple minor other things.

* Cleaning up around case-insensitive hits for `[^a-z]py.*2` in tests/.

* Removing obsolete Python 2 mention in compiling.rst

* Proper `#error` for Python 2.

* Using PY_VERSION_HEX to guard `#error "PYTHON 2 IS NO LONGER SUPPORTED.`

* chore: bump pre-commit

* style: run pre-commit for pyupgrade 3+

* tests: use sys.version_info, not PY

* chore: more Python 2 removal

* Uncommenting Centos7 block (PR #3691 showed that it is working again).

* Update pre-commit hooks

* Fix pre-commit hook

* refactor: remove Python 2 from CMake

* refactor: remove Python 2 from setup code

* refactor: simplify, better static typing

* feat: fail with nice messages

* refactor: drop Python 2 C++ code

* docs: cleanup for Python 3

* revert: intree

revert: intree

* docs: minor touchup to py2 statement
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

* Rollback of PR #3030 (Working around Centos 8 failure).

* Adding `-DCMAKE_BUILD_TYPE=Release`

* Improving existing workaround (undoing the originally intended rollback).

* Fixing minor documentation bug.

Sometimes programmers want to control this, and while it can be changed after the fact, it's commonly set via a CMAKE_ variable; if that variable is set, we should respect that (like the CMAKE_INTERPROCEDURAL_OPTIMIZATION setting).

* feat: import check as a common function

* docs: add cmake to docs

* fix: reduce target collision in add_submodule mode

Closes #2420

* fix: update CMakeLists.txt

* 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: include PYTHON_IS_DEBUG

This variable is a CMake community standard to set the C++
standard of a build. Document it in favor of the previous variable,
which stays as a legacy flag for existing projects.

https://cmake.org/cmake/help/v3.17/variable/CMAKE_CXX_STANDARD.html

Currently select_cxx_standard(), which sets PYBIND11_CPP_STANDARD when
not externally set, is only called from pybind11_add_module(), but the
embed target setup (which runs unconditionally) makes use of
${PYBIND11_CPP_STANDARD}, which isn't set yet.  This commit removes the
`select_cxx_standard` function completely and just always runs the
standard detection code.

This also tweaks the detection code to not bothering checking for the
`-std=c++11` flag when the `-std=c++14` detection succeeded.

All targets provided by pybind11:

* pybind11::module - the existing target for creating extension modules
* pybind11::embed - new target for embedding the interpreter
* pybind11::pybind11 - common "base" target (headers only)

Makes room for an eventual pybind11::embedded target.