Release PDB2PQR 3.6.1

[sobolevnrm]

• [x] Fix documentation build problem #348
• [x] Fix copyright issue #344
• [x] Update versions in PDB2PQR config files
• [x] Update versions in PDB2PQR documentation
• [x] Check that the documentation still works
• [x] Tag and release on GitHub
• [x] Release PyPI package

[sobolevnrm]

See https://pypi.org/project/pdb2pqr/3.6.1/