The PyEval_InitThreads() and PyEval_ThreadsInitialized() functions are
now deprecated and will be removed in Python 3.11. Calling
PyEval_InitThreads() now does nothing. The GIL is initialized by
Py_Initialize() since Python 3.7.

Closes #2270

* added overload for l-value ref-qualified methods

* Added test.
Before, the code would have failed to build.

* (docs) convert note to real note

* (docs) Add information about (deep)copy

For rationale, see #2241, eeb10448;
there is a second entry point function defined by the PYBIND11_MODULE
macro that also needs to be annotated as unused.

This change defines a new, portable macro PYBIND11_MAYBE_UNUSED to
mark declarations as unused, and annotates the PYBIND11_MODULE entry
point with this attribute.

The purpose of this annotation is to facilitate dead code detection,
which might otherwise consider the module entry point function dead,
since it isn't otherwise used. (It is only used via FFI.)

Add warnings about extending STL

Primarily for the ccmake curses GUI

* Test on Python 3.9 beta
* Pin Sphinx
* Newer version of PyPy

This reverts commit 9ed8b440.

* Add AutoWIG to list of binding generators

This adds support for a `py::args_kw_only()` annotation that can be
specified between `py::arg` annotations to indicate that any following
arguments are keyword-only.  This allows you to write:

    m.def("f", [](int a, int b) { /* ... */ },
          py::arg("a"), py::args_kw_only(), py::arg("b"));

and have it work like Python 3's:

    def f(a, *, b):
        # ...

with respect to how `a` and `b` arguments are accepted (that is, `a` can
be positional or by keyword; `b` can only be specified by keyword).

- Not currently supported on PyPy

__init__(self) cannot return values. According to https://stackoverflow.com/questions/2491819/how-to-return-a-value-from-init-in-python __new__(cls) should be used, which works.