Special searching for backward counterparts for circular routes

[alexey-zakharenkov]

Solves #46.

Indeed, before Nov 27, 2023 the validator generated spurious notice _"Only one route in routemaster. Please check if it needs a return route" for circular routes that started at different stations (like A-B-C-D-A and B-A-D-C-B) as if they had no return direction. After #44 the notice vanished, but it vanished at all for circular routes.

In this PR, the checking for return direction of circular routes is resumed, fixed and supplemented with tests.

[SXHRYU]

And add .venv to .gitignore