Fix encode host

[robinaly]

Summary

The encode_host method used the regular expression in the variable IPv4_STYLE_HOSTNAME. This was before:
r"^[0-9]+.[0-9]+.[0-9]+.[0-9]+$" which wrongly allows hostnames like 023b76x43144 to pas as a ipv4 address. This is now fixed by changed the url to r"^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$"

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.: Not applicable IMHO