AttributeError: _ElementInterface instance has no attribute 'nsmap'

As a followup to #823, the above error is occurring when building on RHEL 6. It appears that the `nsmap` attribute is `lxml`-specific. More than that, it seems that in the version available, it seems that `nsmap` can only be set in the constructor. There is a `_nsmap` attribute, but it's probably supposed to be private.

So in summary, in order to continue using `nsmap`, we'd need to have a hard dependency on `lxml`, plus some kind of patch for the older versions.