fix: accept bracketed IPv6 netlocs in domain_filter

[santhreal]

domain_filter accepted bare IPv6 literals like ::1 but rejected the bracketed netloc urlsplit produces for a normal IPv6 URL. check_url therefore returned None for every properly formed IPv6 HTTP URL, while IPv4 peers passed.

Accept [addr] and [addr]:port with the same port rules as IPv4.

from courlan import check_url
check_url("http://[2001:db8::1]/page")
# was None

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (6f0f507) to head (1f5de32).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #144   +/-   ##
=======================================
  Coverage   99.77%   99.78%           
=======================================
  Files          14       14           
  Lines         905      915   +10     
  Branches      173      176    +3     
=======================================
+ Hits          903      913   +10     
  Misses          2        2           

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