v0.10.3

Changes:

• Update central dependencies optimade (v0.12.1) and aiida-core (v1.4.1) (#139).
• Add JSON config files to MANIFEST.in, including them in the source distribution (#134).
• Fallback to config.json if OPTIMADE_CONFIG_FILE environment variable is not defined or points to a non-existing file (#134).

Developer:

• Update CI GitHub Actions workflow (#133, #139)
  The workflow is more stream-lined and shouldn't fail as often due to bad hooks.
• Update use of # pragma: no cover (#134).