[Python] Fix assignment after line continuation (#3940)

Fixes #3939

This commit ensures to pop `line-continuation-body` off stack
immediately after consuming a string.

To achieve that, all exiting `string` related contexts are converted
to immediatelly pop themself off stack and new gatekeeper contexts
are created to push those on stack.

As a positive side-effect syntax cache size is reduced from 546kB
to about 477kB and parsing performance seems to be slightly better.