[BUG] SIGSEGV if Py_Initialize not called when Shared Library instead of Module

It is stated in https://pybind11.readthedocs.io/en/stable/compiling.html#pybind11-add-module that one can use a MODULE or a SHARED keyword when building a target in CMake.

I wanted to create a SHARED library with a class handling all the python binding stuff of a bigger project. When calling from the bigger project the classes and Python methods from the shared library, I got this SIGSEGV:

#8  0x00007fe33f23a558 in PyUnicode_InternInPlace () from /usr/lib/x86_64-linux-gnu/libpython3.6m.so.1.0
#9  0x00007fe33f23a62d in PyUnicode_InternFromString () from /usr/lib/x86_64-linux-gnu/libpython3.6m.so.1.0
#10 0x00007fe33f1e012c in PyImport_Import () from /usr/lib/x86_64-linux-gnu/libpython3.6m.so.1.0
#11 0x00007fe33f1e027a in PyImport_ImportModule () from /usr/lib/x86_64-linux-gnu/libpython3.6m.so.1.0
#12 0x00007fe342f6edf5 in pybind11::module_::import (name=0x7fe342f83aab "os") at /usr/local/include/pybind11/pybind11.h:992
#13 0x00007fe342f62a97 in USBTMC::USBTMC (this=0x7fffbd6e011f) at usb/USBTMC.cpp:15
#14 0x000055fb3bde1f56 in main (argc=1, argv=0x7fffbd700dd8) at main.cxx:57

I fixed the crash by calling Py_Initialize() in the shared library class constructor, following this link https://stackoverflow.com/questions/53383322/pydict-setitemstring-segfaults

I would like to propose that this segfault is prevented on pybind11 side by checking the result of Py_IsInitialized(), with a warning error telling user to call Py_Initialize. And/or that further information is given on this link https://pybind11.readthedocs.io/en/stable/compiling.html#pybind11-add-module about how to correctly use pybind when using SHARED instead of MODULE.

Thanks for this nice package!!

Start with the pybind11::module_::import call at pybind11.h:992 and the documented pybind11_add_module CMake guidance. Reproduce the SHARED-library path described from USBTMC.cpp:15 and main.cxx:57, then evaluate handling of an uninitialized interpreter using Py_IsInitialized and the corresponding documentation update. Done means the reported SIGSEGV path is addressed or the SHARED-library initialization requirements are clearly documented.