pythonPackages.html5lib_0_9999999: Fix build

Regression introduced by 345b35c4.

The commit in question switched the implementation of buildPythonPackage
so that .override now is behaving the same way as for other packages
that are using callPackage.

So the fix for html5lib_0_9999999 is to just use .overridePythonAttrs
instead of .override.

Note that the mentioned commit initially introduced this as
overridePythonPackage but it got later renamed to overridePythonAttrs in
abdb58e4

.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>