FIX: Use openpyxl instead of xlrd

Fixes #29 (closed)

Summary

Removes the dependency on the discontinued xlrd<=1.2, uses xlrd>=2.0 for xls files and openpyxl for xlsx files.

Focus

Apparently we waited long enough so that Pandas fixed their engines for reading xls(x) files when both xlrd and openpyxl are installed. So only added a unittest for xls files and removed the version limitation

Test Environment

Simply do the unittests and the integrationtests with the updated xlrd version.

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