GMSO 0.8.0

Breaking change

• Stop testing Python 3.6 and add testing for Python 3.9 by @umesh-timalsina in #633

Features

• Add combining rule to ForceField by @umesh-timalsina in #632
• Prevent accidental defaults values while creating Potentials by @umesh-timalsina in #635
• Update parmed converter to use new residue infrastructure by @daico007 in #613

Bug fixes

• Fix bugs in convert_foyer_xml by @daico007 in #629, #636

Maintenance

• [pre-commit.ci] pre-commit autoupdate by @pre-commit-ci in #630

Full Changelog: 0.7.3...0.8.0