Uses deprecated Py_UNICODE API and fails to build with GCC 14

[glaubitz]

Trying to build metamagic.json with GCC 14 as the default C/C++ compiler fails with:

[   13s] gcc -fno-strict-overflow -Wsign-compare -DNDEBUG -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -g -DOPENSSL_LOAD_CONF -fwrapv -fno-semantic-interposition -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -g -IVendor/ -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -g -IVendor/ -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g -fPIC -I/usr/include/python3.12 -c metamagic/json/_encoder/_encoder.c -o build/temp.linux-x86_64-cpython-312/metamagic/json/_encoder/_encoder.o -O3
[   13s] metamagic/json/_encoder/_encoder.c:53:31: warning: missing braces around initializer [-Wmissing-braces]
[   13s]    53 | PyTypeObject PyEncoder_Type = {
[   13s]       |                               ^
[   13s] In file included from /usr/include/python3.12/Python.h:44,
[   13s]                  from metamagic/json/_encoder/_encoder_buffer.h:11,
[   13s]                  from metamagic/json/_encoder/_encoder.c:8:
[   13s] /usr/include/python3.12/object.h:142:9: error: initialization of ‘long int’ from ‘void *’ makes integer from pointer without a cast [-Wint-conversion]
[   13s]   142 |         (type)                   \
[   13s]       |         ^
[   13s] metamagic/json/_encoder/_encoder.c:54:5: note: in expansion of macro ‘PyObject_HEAD_INIT’
[   13s]    54 |     PyObject_HEAD_INIT(NULL)
[   13s]       |     ^~~~~~~~~~~~~~~~~~
[   13s] /usr/include/python3.12/object.h:142:9: note: (near initialization for ‘PyEncoder_Type.ob_base.ob_size’)
[   13s]   142 |         (type)                   \
[   13s]       |         ^
[   13s] metamagic/json/_encoder/_encoder.c:54:5: note: in expansion of macro ‘PyObject_HEAD_INIT’
[   13s]    54 |     PyObject_HEAD_INIT(NULL)
[   13s]       |     ^~~~~~~~~~~~~~~~~~
[   13s] metamagic/json/_encoder/_encoder.c:53:31: warning: missing braces around initializer [-Wmissing-braces]
[   13s]    53 | PyTypeObject PyEncoder_Type = {
[   13s]       |                               ^
[   13s] metamagic/json/_encoder/_encoder.c:53:31: warning: missing braces around initializer [-Wmissing-braces]
[   13s] metamagic/json/_encoder/_encoder.c: In function ‘encode_string’:
[   13s] metamagic/json/_encoder/_encoder.c:873:21: error: implicit declaration of function ‘PyUnicode_GET_SIZE’; did you mean ‘PyDict_GET_SIZE’? [-Wimplicit-function-declaration]
[   13s]   873 |     input_size    = PyUnicode_GET_SIZE(pystr);
[   13s]       |                     ^~~~~~~~~~~~~~~~~~
[   13s]       |                     PyDict_GET_SIZE
[   13s] metamagic/json/_encoder/_encoder.c:874:21: error: implicit declaration of function ‘PyUnicode_AS_UNICODE’; did you mean ‘PyUnicode_AsUCS4’? [-Wimplicit-function-declaration]
[   13s]   874 |     input_unicode = PyUnicode_AS_UNICODE(pystr);
[   13s]       |                     ^~~~~~~~~~~~~~~~~~~~
[   13s]       |                     PyUnicode_AsUCS4
[   13s] metamagic/json/_encoder/_encoder.c:874:19: error: assignment to ‘Py_UNICODE *’ {aka ‘int *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
[   13s]   874 |     input_unicode = PyUnicode_AS_UNICODE(pystr);
[   13s]       |                   ^
[   13s] metamagic/json/_encoder/_encoder.c: In function ‘encode_json’:
[   13s] metamagic/json/_encoder/_encoder.c:903:19: error: assignment to ‘Py_UNICODE *’ {aka ‘int *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
[   13s]   903 |     input_unicode = PyUnicode_AS_UNICODE(pystr);
[   13s]       |                   ^
[   13s] In file included from metamagic/json/_encoder/_encoder.c:12:
[   13s] metamagic/json/_encoder/_encoder.h: At top level:
[   13s] metamagic/json/_encoder/_encoder.h:25:22: warning: ‘PyType_BaseEncoder’ defined but not used [-Wunused-variable]
[   13s]    25 | static PyTypeObject* PyType_BaseEncoder;
[   13s]       |                      ^~~~~~~~~~~~~~~~~~
[   13s] error: command '/usr/bin/gcc' failed with exit code 1

Since Py_UNICODE is a deprecated API (see: https://peps.python.org/pep-0624/), its use should be replaced to fix this bug.