cext: add Python 3.10 support
* Replace "Py_TYPE(obj) = type;" with "Py_SET_TYPE(obj, type);"
* Add pythoncapi_compat.h header file to get Py_SET_TYPE() on Python
2.7-3.8. Header file added to mercurial/ and
contrib/python-zstandard/zstd/common/.
In Python 3.10, Py_TYPE(obj) must not longer be used as an l-value.
pythoncapi_compat.h comes from:
https://github.com/pythoncapi/pythoncapi_compat
Differential Revision: https://phab.mercurial-scm.org/D9825
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']