ElementInclude false detection of recursive include

[Rainer Hausdorf]

Python : sys.version_info(major=3, minor=7, micro=3, releaselevel='final', serial=0)
lxml.etree : (4, 3, 4, 0)
libxml used : (2, 9, 9)
libxml compiled : (2, 9, 9)
libxslt used : (1, 1, 33)
libxslt compiled : (1, 1, 33)

In some cases ElementInclude of lxml does raise FatalIncludeError because of recursive include detection, when the same file gets included multiple times, but not recursive. I provided a minimal example. The xinclude does work.

[scoder (scoder)]

PR welcome that improves the include handling.

[Rainer Hausdorf (rhx84)]

Provided PullRequest: https://github.com/lxml/lxml/pull/286

[scoder (scoder)]

https://github.com/lxml/lxml/pull/286