Junctions always involve a 90 degree turn, so we only need
to check two directions.

Flatten the grid into a single string (separated by line-feeds to
side-step wrap-around problems) and use the offset in the string
to identify cells instead of (x, y) tuples.

Don't calculate part one separately, just do it as we go.

No point checking for connected nodes in +x or +y directions,
as we haven't processed them yet.

I think only two repeats of the reductions are ever needed,
so ditch tracking whether the counts are modified or not.

Golf away a couple of other minor lines.

And Python3 fixes....