feat(allow_hosts): add support for hostname resolution

[Jguer]

Hey, thanks for the nice project.

Scenario I'm attempting to solve:

• We have a service in our project we must connect to from a docker-composed application.
• The hostname is stable but not the IP address.

Somewhat similar to #73

What does this MR do

• Resolve allowed hostnames into IPs if possible and check if added exceptions are correct IPv4 addresses
• Strip strings added to the list to make sure pytest --allow-hosts="172.18.0.2, 172.18.0.1" works

Questions

• no checks for IPv6 addresses was done, is it relevant to readd?

If type hinting is necessary or some documentation needs to be changed feel free to ask.

[qlty-cloud-legacy]

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[tonycosentini]

Any way this can get into a release? I think it would help a lot if you want to use this plugin with something like docker-compose.

[mgaitan]

please, could we upgrade this PR and #73 and get them into a release? I could help if you give me instructions.

[miketheman]

Closed via #189