Rewrite scripts/parallel-nextpnr-nexus in Python

[alanvgreen]

scripts/parallel-nextpnr-nexus is written in a modern dialect of bash that is not widely supported. This causes the check-multi-proj CI test to break badly, though it is currently marked as optional because even single threaded it does not always pass.

For better portability and long term maintainbility, this script ought to be rewritten in Python.

To do

1. Revert #251
2. Rewrite in Python

#251 #279

[alanvgreen]

Revert is in #282