Use Protocols to annotate python3 <= 3.6 `compileall` argument

[SKalt]

in stdib/3/compileall.pyi, there's a comment

# rx can be any object with a 'search' method; once we have Protocols we can change the type.

Since we have Protocols, we can do this.

history

The comment was introduced on Mar 18, 2017 by @JelleZijlstra in commit efdf2f1, and came up in the review of #3476.

[JelleZijlstra]

Looks like #5172 fixed this