Change NAMESPACE_* macros into PYBIND11_NAMESPACE_* (#2283)

* Change NAMESPACE_BEGIN and NAMESPACE_END macros into PYBIND11_NAMESPACE_BEGIN and PYBIND11_NAMESPACE_END

* Fix sudden HomeBrew 'python not installed' error

* Sweep difference in 'Class.__init__() must be called when overriding __init__' error message between CPython and PyPy under the rug

* Homebrew updated to 3.8 yesterday.

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