Revert "Add q3py.pc pkg-config file"

This reverts commit c7f7766d15e50cd631c30cd48a58134670d781fa.

Users of q3py should include its public header and not link with
q3py directly. Instead, the capsule and dynamic loading machinery
of Python shall be used.
Revert "Make Python version available as autoconf substitution"

This reverts commit f352ffff002780fa6f64238de5d0f8d73b785624.

This was mainly needed for the q3py.pc.in pkg-config template,
which is now deprecated.