Add support for SIP v5 bindings

SIP v5 and above rely on a PEP-0518 based tooling system to build
bindings (usually distutils). v4, as our currently supported generator,
is only a dummy tool -- it relies on the user to compile and link the
bindings.

This commit adds support for SIP v5.

- I've added a "sip-generate" tool, that differs from the standard
sip-build in that it only generates the binding glue; this relies on a
private, undocumented method of the sipbuild.Builder class.

- The cmake/modules directory now has a pyproject.toml skeleton
manifest, which is processed by the new add_sip_python_module_v5 macro.
It works identically to the v4 macro, but targets the sip-generate tool.
Compilation happens in the exact same was as we do with v4.

Key remarks:

- All the data must be now injected in the skeleton manifest (as opposed
to build flags in v4).
- SIP v5 makes use of protected Qt methods; for this reason, it supplies
a "protected-as-public" override, which I emulate in the
add_sip_python_module_v5 macro.
- Like v4, v5 also needs to be told the fully qualified name of the SIP
package (e.g. `import PyQt5.sip` -- PyQt5.sip is the value). Otherwise,
the native library will not find the package and the initialization will
fail.
- The initialization function changes, from "PyInit_pykrita" to
"PyInit_krita" (the innermost module name). This has been macro'd for
backwards compatibility.

BUG: 415743

CCMAIL: kimageshop@kde.org