MAINT: Remove usage of the PEP 604 pipe operator

[BvB93]

Remove the PEP 604 pipe operator and replace it with typing.Union.

The pipe operator is currently not yet universally supported by all type checkers (see python/typeshed#4819).
While we don't necessarily have to wait for all type checkers to support it, I feel that delaying its usage to numpy 1.22
would still be a reasonable compromise.

Examples

from __future__ import annotations

# Equivalent to `def func(a: Union[int, float]) -> bool: ...`
def func(a: int | float) -> bool: ...   

[charris]

Thanks Bas.