MAINT: Add const qualifiers to char* types in pycapsule.pxd.

Eliminate C compiler warnings by making the constness of the
char* pointers in the declarations in pycapsule.pxd match
the actual types of the PyCapsule_* functions.

Use Py_TYPE() and __Pyx_SET_SIZE() functions

Replace direct access to PyObject members with functions like
Py_TYPE() and __Pyx_SET_SIZE().

Change suggested by Stepan Kasal on Cython-devel list

The C-API function applied in previous commit (https://github.com/cython/cython/commit/b37607f46b165e81f7af2b4eec2ae7f76f7717df) is now also obsolete.
This patch works for cython 0.29 and cpython after (https://github.com/python/cpython/commit/1e996c3a3b51e9c6f1f4cea8a6dbcf3bcb865060#diff-d2c90d06d281ca8dd7ac8b90109fc26755f7319dd209b5f9416efdecfa56e289).

Fix "__Pyx_PyObject_AsDouble()" special-casing to prevent de-optimisation through potential PyFloat unpack-repack cycles.

(Probably not as optimal since it can lead to de-optimisation when we actually need a PyFloat result.)

Add flags and missing exception declaration for PyOS_*() string conversion functions in cpython/conversion.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.9/c-api/file.html).

PyObject_AsFileDescriptor() is already present in object.pxd.

Closes https://github.com/cython/cython/pull/3902