PyGC_HEAD is private in Python 3.9

As of Python 3.9, PyGC_HEAD (which meliae relies on) is private.

See https://bugs.python.org/issue40241 for background

Compilation error:

creating build/temp.linux-x86_64-3.9
creating build/temp.linux-x86_64-3.9/meliae
x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/python3.9 -c meliae/_scanner.c -o build/temp.linux-x86_64-3.9/meliae/_scanner.o
meliae/_scanner.c: In function ‘__pyx_pymod_exec__scanner’:
meliae/_scanner.c:3435:46: error: ‘PyGC_Head’ undeclared (first use in this function)
 3435 | __pyx_t_1 = __Pyx_PyInt_FromSize_t((sizeof(PyGC_Head))); if (unlikely(!__pyx_t_1)) __PYX_ERR(0, 52, __pyx_L1_error)
      | ^~~~~~~~~
meliae/_scanner.c:3435:46: note: each undeclared identifier is reported only once for each function it appears in
error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1

An obvious fix would be to import PyGC_HEAD from internal/pycore_gc.h, but that would leave meliae open to ABI changes between minor releases.