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First: This is the bug:
In [1]: 'großhandel-shop'.encode('idna')
Out[1]: b'grosshandel-shop'
This lead to this:
'xn--einla-pqa'.decode('idna')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python3.6/encodings/idna.py", line 214, in decode
 result.append(ToUnicode(label))
 File "/usr/lib/python3.6/encodings/idna.py", line 139, in ToUnicode
 raise UnicodeError("IDNA does not round-trip", label, label2)
UnicodeError: ('IDNA does not round-trip', b'xn--einla-pqa', b'einlass')
https://stackoverflow.com/questions/9806036/idna-does-not-round-trip
xn--grohandel-shop-2fb has been correctly encoded by IDNA 2008 (which is correct in Germany/DENIC since a while). Your Python very likely tries to decode it using the old IDNA 2003, which doesn't know 'ß'. see denic.de/en/know-how/idn-domains