Loosen xlrd requirement

[znicholls]

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Description of PR

Loosen the pinning of xlrd. The pin isn't needed and it's causing issues for downstream users (e.g. attempts to make openscm-runner a conda package https://github.com/conda-forge/staged-recipes/pull/15573#issuecomment-898890003). It would be great if these changes could be propagated to conda and pypi fairly quickly as a bug fix.

[codecov[bot]]

Codecov Report

Merging #572 (7fd9c52) into main (bd6fb43) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #572   +/-   ##
=====================================
  Coverage   93.7%   93.7%           
=====================================
  Files         50      50           
  Lines       5332    5334    +2     
=====================================
+ Hits        4997    4999    +2     
  Misses       335     335           

Impacted Files	Coverage Δ
setup.py	0.0% <ø> (ø)
pyam/core.py	94.3% <100.0%> (ø)
pyam/utils.py	91.6% <100.0%> (+<0.1%)	:arrow_up:
tests/test_io.py	100.0% <100.0%> (ø)

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[znicholls]

This seems to not work with xlrd==2.0.0 - maybe require xlrd>=2.0.1 instead?

Had to make some tweaks to make sure engine passing worked, let's see what the CI says