Write single bonds to chem_comp_bond as fallback

[padix-key]

When calling biotite.structure.io.pdbx.set_structure() with intra-residue bonds being Bond.ANY or Bond.COORDINATION, it will error, as chem_comp_bond has no counterpart for these bond types. With this fix set_structure() will emit a warning and assign a single bond instead.

[codspeed-hq]

Merging this PR will not alter performance

✅ 63 untouched benchmarks

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_{Comparing padix-key:fallback-bonds (4e58775) with main (2018578)}