Support Python 3.13

[erwald]

Currently installation fails on Python 3.13, apparently due to msgspec, which itself has not migrated yet:

$ python --version
Python 3.13.1

$ pip install squigglepy
...
Building wheels for collected packages: msgspec
  Building wheel for msgspec (pyproject.toml) ... error
  error: subprocess-exited-with-error
  
  × Building wheel for msgspec (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [51 lines of output]
      running bdist_wheel
      running build
      running build_py
      creating build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/msgpack.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/_version.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/__init__.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/yaml.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/structs.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/inspect.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/_json_schema.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/json.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/_utils.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/toml.py -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/msgpack.pyi -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/__init__.pyi -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/json.pyi -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/structs.pyi -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      copying msgspec/py.typed -> build/lib.macosx-14.5-x86_64-cpython-313/msgspec
      UPDATING build/lib.macosx-14.5-x86_64-cpython-313/msgspec/_version.py
      set build/lib.macosx-14.5-x86_64-cpython-313/msgspec/_version.py to '0.18.6'
      running build_ext
      building 'msgspec._core' extension
      creating build/temp.macosx-14.5-x86_64-cpython-313/msgspec
      clang -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -I/Users/username/.pyenv/versions/3.13.1/include/python3.13 -c msgspec/_core.c -o build/temp.macosx-14.5-x86_64-cpython-313/msgspec/_core.o
      msgspec/_core.c:500:13: error: call to undeclared function '_PyUnicode_EQ'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
              if (_PyUnicode_EQ(kwname, key)) {
                  ^
      msgspec/_core.c:4452:12: error: call to undeclared function '_PySet_NextEntry'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
          while (_PySet_NextEntry(state->structs_set, &set_pos, &set_item, &set_hash)) {
                 ^
      msgspec/_core.c:7316:17: error: call to undeclared function '_PyUnicode_EQ'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                  if (_PyUnicode_EQ(kwname, field)) {
                      ^
      msgspec/_core.c:7723:17: error: call to undeclared function '_PyUnicode_EQ'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
                  if (_PyUnicode_EQ(kwname, field)) goto kw_found;
                      ^
      msgspec/_core.c:11254:72: error: too few arguments to function call, expected 6, have 5
          int out = _PyLong_AsByteArray((PyLongObject *)int128, buf, 16, 0, 0);
                    ~~~~~~~~~~~~~~~~~~~                                      ^
      /Users/username/.pyenv/versions/3.13.1/include/python3.13/cpython/longobject.h:111:17: note: '_PyLong_AsByteArray' declared here
      PyAPI_FUNC(int) _PyLong_AsByteArray(PyLongObject* v,
                      ^
      msgspec/_core.c:12426:12: error: call to undeclared function '_PySet_NextEntry'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
          while (_PySet_NextEntry(obj, &ppos, &item, &hash)) {
                 ^
      msgspec/_core.c:13748:12: error: call to undeclared function '_PySet_NextEntry'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
          while (_PySet_NextEntry(obj, &ppos, &item, &hash)) {
                 ^
      7 errors generated.
      error: command '/usr/bin/clang' failed with exit code 1
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
  ERROR: Failed building wheel for msgspec
Failed to build msgspec
ERROR: ERROR: Failed to build installable wheels for some pyproject.toml based projects (msgspec)

Options include (1) wait for msgspec to release a new version supporting Python 3.13, or (2) factor msgspec out of the codebase.