Fix safe_url_string to preserve IPv6 brackets in netloc

[danishashko]

Fixes #193

Problem

safe_url_string manually rebuilds the netloc component from the parts returned by urlsplit(). The hostname attribute always strips square brackets from IPv6 addresses (e.g. [2402:4e00:40:40::2:3b6] becomes 2402:4e00:40:40::2:3b6). The brackets were not restored when rebuilding netloc_bytes, so the reconstructed URL was malformed.

On subsequent calls (e.g. inside Scrapy middlewares reconstructing a Request from the stored URL), urlsplit() on the bracket-less URL parsed the first IPv6 segment as the host and the remainder as a port, raising ValueError: Port could not be cast to integer value.

Fix

Detect IPv6 addresses by checking for : in the hostname and restore the brackets before appending to netloc_bytes. IPv4 addresses and DNS hostnames never contain :, so the check is unambiguous.

Tests

• Removed http://[2a01:5cc0:1:2::4] from KNOWN_SAFE_URL_STRING_URL_ISSUES (the xfail for this issue) since it now passes.
• Added four new parametrized cases: the address from the bug report with and without a port, plus ::1 with and without a port and path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.97%. Comparing base (61db33b) to head (a396d67).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #253   +/-   ##
=======================================
  Coverage   97.96%   97.97%           
=======================================
  Files           9        9           
  Lines         491      493    +2     
  Branches       83       84    +1     
=======================================
+ Hits          481      483    +2     
  Misses          6        6           
  Partials        4        4           

Files with missing lines	Coverage Δ
w3lib/url.py	99.57% <100.00%> (+<0.01%)	⬆️

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[wRAR]

Thanks!