xlrd<2 does not work with python3.9, so pinning it won't help

[berndf]

Hello, referring to #41 and #45 here - xlrd 1.2.0 results in this with python 3.9:

AttributeError: 'ElementTree' object has no attribute 'getiterator'

Therefore I'd vote to not advertise xlsx support if xlrd>=2 is installed, rather than force an outdated dependency.

[chfw]

hmm.. indeed, I can do such a check before the plugin registration.