pywemo fails to build after libxml2 upgrade

[mweinelt]

Describe the bug
After d01a4e3 pywemo fails a test.

_________________________ test_bridge_getdevicestatus __________________________

bridge = <WeMo Bridge "WeMo Device", Lights: 1, Groups: 0>

    @pytest.mark.vcr()
    def test_bridge_getdevicestatus(bridge):
        status = bridge.bridge_getdevicestatus(LIGHT_ID)
        expected = b''.join(
            [
                b'<DeviceStatus>',
                b'<IsGroupAction>NO</IsGroupAction>',
                b'<DeviceID available="YES">0017880108DA898B</DeviceID>',
                b'<CapabilityID>10006,10008,30008,30009,3000A</CapabilityID>',
                b'<CapabilityValue>0,255:0,,,</CapabilityValue>',
                b'<LastEventTimeStamp>0</LastEventTimeStamp>',
                b'</DeviceStatus>',
            ]
        )
>       assert et.tostring(status) == expected
E       AssertionError: assert b'<DeviceStat...ceStatusList>' == b'<DeviceStat...DeviceStatus>'
E         Use -v to get the full diff

tests/ouimeaux_device/test_bridge.py:53: AssertionError

To Reproduce
Steps to reproduce the behavior:

1. nix-build -A python3Packages.pywemo

Notify maintainers
@fabaff

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: python3Packages.pywemo
# a list of nixos modules affected by the problem
module:

[mweinelt]

Reported upstream here pywemo/pywemo#268