v0.18.0

Major updates:

• Dropped Python 3.6 support, added Python 3.9 support (#232).
• Update the AiiDA filter transformer class and entry collections to be compatible with v0.16.0 of optimade (#252).
• Update to AiiDA v1.6 (#232, #236, #252).

Developer updates:

• Several minor code optimizations to use more of the upstream optimade code (#252, #262).
• Several dependency updates (#226, #232, #235, #236, #247, #248, #252).
• Update to full version GH Actions (#243).
• Remove use of CODECOV_TOKEN (#234).