Union types detection disabled by default

[netromdk]

Since version 1.5.2, union types detection has been disabled by default.

The caveat section explains:

Detecting union types (X | Y PEP 604) can be tricky because Vermin doesn't know all underlying details of constants and types since it parses and traverses the AST. For this reason, heuristics are employed and this can sometimes yield incorrect results (#103). To enable (unstable) union types detection, use --feature union-types.