gh-91603: Speed up operator "|" for UnionType (GH-91955)

Reduce the complexity from O((M+N)^2) to O(M*N), where M and N are the length
of __args__ for both operands (1 for operand which is not a UnionType).

As a consequence, the complexity of parameter substitution in UnionType has
been reduced from O(N^3) to O(N^2).

Co-authored-by: Yurii Karabas <1998uriyyo@gmail.com>