compilation fails with cpython master due to removed function

[tacaswell]

Issue description

Use with cpython master branch (of scipy) fails with

  creating build/temp.linux-x86_64-3.9/scipy/fft/_pocketfft
    compile options: '-DPOCKETFFT_PTHREADS -I/home/tcaswell/.virtualenvs/bleeding/include/site/python3.9 -I/home/tcaswell/.virtualenvs/bleeding/include -I/home/tcaswell/.pybuild/bleeding/include/python3.9 -c'
    extra options: '-std=c++14 -fvisibility=hidden'
    g++: scipy/fft/_pocketfft/pypocketfft.cxx
    In file included from scipy/fft/_pocketfft/pypocketfft.cxx:15:
    /home/tcaswell/.virtualenvs/bleeding/include/site/python3.9/pybind11/pybind11.h: In member function ‘void pybind11::gil_scoped_acquire::dec_r
    /home/tcaswell/.virtualenvs/bleeding/include/site/python3.9/pybind11/pybind11.h:1953:13: error: ‘PyThreadState_DeleteCurrent’ was not declare
     1953 |             PyThreadState_DeleteCurrent();
          |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
          |             PyThreadState_Delete
    error: Command "g++ -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -DPOCKETFFT_PTHREADS -I/home/tcaswell/.virthon3.9/site-packages/numpy/core/include -I/home/tcaswell/.virtualenvs/bleeding/include -I/home/tcaswell/.pybuild/bleeding/include/python3.9 -c s/_pocketfft/pypocketfft.o.d -std=c++14 -fvisibility=hidden" failed with exit status 1

This is due to cpython removing this function for py3.9 : python/cpython#15315 / https://bugs.python.org/issue37878

This function is used at

pybind11/include/pybind11/pybind11.h

Lines 1939 to 1957 in f3109d8

	PYBIND11_NOINLINE void dec_ref() {
	--tstate->gilstate_counter;
	#if !defined(NDEBUG)
	if (detail::get_thread_state_unchecked() != tstate)
	pybind11_fail("scoped_acquire::dec_ref(): thread state must be current!");
	if (tstate->gilstate_counter < 0)
	pybind11_fail("scoped_acquire::dec_ref(): reference count underflow!");
	#endif
	if (tstate->gilstate_counter == 0) {
	#if !defined(NDEBUG)
	if (!release)
	pybind11_fail("scoped_acquire::dec_ref(): internal error!");
	#endif
	PyThreadState_Clear(tstate);
	PyThreadState_DeleteCurrent();
	PYBIND11_TLS_DELETE_VALUE(detail::get_internals().tstate);
	release = false;
	}
	}

Reproducible example code

Trying to install scipy from source with cpython master.

[wjakob]

I opened a ticket on the Python bug tracker: https://bugs.python.org/issue38266

[bstaletic]

CPython "undeleted" this function.