bpo-38693: Use f-strings instead of str.format() within importlib (#17058)

This is a small performance improvement, especially for one or two hot
places such as _handle_fromlist() that are called a lot and the
.format() method was being used just to join two strings with a dot.

Otherwise it is merely a readability improvement.

We keep `_ERR_MSG` and `_ERR_MSG_PREFIX` as those may be used elsewhere for canonical looking error messages.