Implement binding to IPv6 addresses in the pytest server fixture.

[tan01]

What do these changes do?

This change allows TestServer to be instantiated with IPv6 hostnames.
Previously, the TestServer would open an IPv4 family socket regardless
of hostname, causing an error to be raised upon starting the server.
The BaseTestServer now parses the hostname and create an IPv4 or
IPv6 socket based on the hostname.

Are there changes in behavior for the user?

Creating a TestServer from test_utils.py with an IPv6 hostname ('::1') no longer throws

 socket.gaierror: [Errno -9] Address family for hostname not supported

Related issue number

None

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[webknjaz]

Plz go through the checklist/add a change fragment.

[tan01]

Went through the checklist, added change fragment/modified contributors.