Requirements & config maintenance release before major refactor

pybibframe is overdue for a major update. Started as a proof of concept for the Library of Congress, and never really got a proper moment to go back & properly engineer. Now's the time, and this release provides a known, last-working checkpoint before embarking on that work.

Remove some dupe code

Changed:

• Move overlapping pipeline action functions to versa

Python 3.7 compatibility

Fixed

• Eliminate use of the asyncio keyword, and eliminate propagation of loop args

Indicator bug fix

Fixes a problem for most uses of the indicator() action function, which didn't match properly on empty indicator values which are stored internally as the empty string instead of how they appear in MARC as a single space.

Enhance marc2bf command line for rapid prototype transform rules

• Add --modfile argument to marc2bf, which allows incorporation of transform rules in a Python file anywhere (without it requiring it to be a module in sys.path)
• Add if_ Versa action function
• Minor bug fixes