automig_sqlite fails without psycopg2

[abe-winter]

• sqlite users shouldn't have to install psyco
• solution: move common code to actual common so automig_sqlite doesn't import migrate_pg
• traceback:

automig_sqlite --glob schema.sql init
Traceback (most recent call last):
  File ".../.direnv/python-3.7.5/bin/automig_sqlite", line 5, in <module>
    from automig.automig_sqlite import main
  File ".../.direnv/python-3.7.5/lib/python3.7/site-packages/automig/automig_sqlite.py", line 5, in <module>
    from .migrate_pg import create_parser, init, update
  File ".../.direnv/python-3.7.5/lib/python3.7/site-packages/automig/migrate_pg.py", line 5, in <module>
    import psycopg2 # pylint: disable=import-error
ModuleNotFoundError: No module named 'psycopg2'