Pybind11

Next release candidate.

* Factory constructor scope fix
* Support for PyPy3 on Windows 32, added to CI
* Fixes for C++20, added to CI for Clang, GCC, and MSVC 2019
* Black formatting for all Python code
* Allow ABI string overrides from 2.4 (advanced)

Next release candidate. Changes since RC 1:

Fixed:

* Python 3.9.0 bug workaround implemented
* Better support in CMake for packing and cross-compiling
* Fix for failing `py::len` leaving error state set
* Intel 18+ explicitly required, an issue with Intel compilers fixed
* Custom ipo flags disabled with `RelWithDebInfo` (use `CMAKE_INTERPROCEDURAL_OPTIMIZATION` for more control)
* Possible duplicate linker flags fixed in `setup_helpers`
* Changes to `py::module_` constructor are no longer provisional.

New features:

* Static typing support for `import pybind11`

This is the first release candidate for pybind11 2.6.0. Please see the new features in the [changelog](https://pybind11.readthedocs.io/en/latest/changelog.html), such as revamped CMake and setuptools support, keyword and positional argument support, and a long list of features, fixes, and improvements. Full Python 3.9 support and enhanced C++20 support.

Known issues:

* `py::module_` direct construction provisionally deprecated
* Removing registered types can cause a segfault - old issue exposed by recent test additions; fix incoming
* On Python 3.9.0 and macOS, pybind11 may segfault when closing the interpreter after running the test suite

v2.6.0 beta release.