feat: setup.py redesign and helpers (#2433)

* feat: setup.py redesign and helpers

* refactor: simpler design with two outputs

* refactor: helper file update and Windows support

* fix: review points from @YannickJadoul

* refactor: fixes to naming and more docs

* feat: more customization points

* feat: add entry point pybind11-config

* refactor: Try Extension-focused method

* refactor: rename alt/inplace to global

* fix: allow usage with git modules, better docs

* feat: global as an extra (@YannickJadoul's suggestion)

* feat: single version location

* fix: remove the requirement that setuptools must be imported first

* fix: some review points from @wjacob

* fix: use .in, add procedure to docs

* refactor: avoid monkeypatch copy

* docs: minor typos corrected

* fix: minor points from @YannickJadoul

* fix: typo on Windows C++ mode

* fix: MSVC 15 update 3+ have c++14 flag

See <https://docs.microsoft.com/en-us/cpp/build/reference/std-specify-language-standard-version?view=vs-2019>

* docs: discuss making SDists by hand

* ci: use pep517.build instead of manual setup.py

* refactor: more comments from @YannickJadoul

* docs: updates from @ktbarrett

* fix: change to newly recommended tool instead of pep517.build

This was intended as a proof of concept; build seems to be the correct replacement.

See https://github.com/pypa/pep517/pull/83

* docs: updates from @wjakob

* refactor: dual version locations

* docs: typo spotted by @wjakob
diff --git a/docs/basics.rst b/docs/basics.rst
index 6bb5f98..71440c9 100644
--- a/docs/basics.rst
+++ b/docs/basics.rst
@@ -11,11 +11,11 @@
 Compiling the test cases
 ========================
 
-Linux/MacOS
+Linux/macOS
 -----------
 
 On Linux  you'll need to install the **python-dev** or **python3-dev** packages as
-well as **cmake**. On Mac OS, the included python version works out of the box,
+well as **cmake**. On macOS, the included python version works out of the box,
 but **cmake** must still be installed.
 
 After installing the prerequisites, run
@@ -138,7 +138,7 @@
 
     $ c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` example.cpp -o example`python3-config --extension-suffix`
 
-For more details on the required compiler flags on Linux and MacOS, see
+For more details on the required compiler flags on Linux and macOS, see
 :ref:`building_manually`. For complete cross-platform compilation instructions,
 refer to the :ref:`compiling` page.