Fixes SSL issues.

Motivation for this change

Before this patch:
linkchecker https://google.com

Fails with this error:

  File "/nix/store/zfs78yih9xm6s9p2n58flz3m0slnmfph-LinkChecker-9.3/lib/python2.7/site-packages/linkcheck/checker/httpurl.py", line 184, in _get_ssl_sock
    line: if raw_connection.sock is None:
    locals:
      raw_connection = <local> None
      raw_connection.sock = <local> !AttributeError: 'NoneType' object has no attribute 'sock'
      None = <builtin> None
AttributeError: 'NoneType' object has no attribute 'sock'

With the patch, it works as expected.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.