1.0.3

Changes include:

• Update documentation to be consistent with the ASDF library documentation.
• Add time-1.2.0 schema to document bugfix requiring additional property to be
  written from asdf-astropy.
• Move packaging to pyproject.toml file from setup.cfg and setup.py
  files.
• Remove tag from within the time-1.1.0 schema.
• Remove tag from within the remaining schemas.