Implement compatibility with pandas 2.0

[danielhuppmann]

Description of PR

This PR implements changes necessary for tests to pass with pandas v2.0 and nbconvert v7.3.

Note that the pytest-legacy workflow and the py3.7-tests (where pandas 2.0 was not released) ensure that also older versions of pandas continue to work with the changes required for compatibility with pandas 2.0.

Closes #738

[codecov]

Codecov Report

Merging #739 (b525775) into main (a87e64e) will increase coverage by 0.0%.
The diff coverage is 94.2%.

@@          Coverage Diff          @@
##            main    #739   +/-   ##
=====================================
  Coverage   95.1%   95.1%           
=====================================
  Files         59      59           
  Lines       6057    6006   -51     
=====================================
- Hits        5761    5713   -48     
+ Misses       296     293    -3     

Impacted Files	Coverage Δ
pyam/figures.py	38.0% <ø> (ø)
tests/test_statistics.py	100.0% <ø> (ø)
tests/test_time.py	100.0% <ø> (ø)
pyam/statistics.py	85.5% <87.5%> (+0.7%)	⬆️
tests/test_tutorials.py	89.4% <92.8%> (-5.2%)	⬇️
pyam/iiasa.py	87.3% <100.0%> (ø)
pyam/utils.py	92.7% <100.0%> (+<0.1%)	⬆️
tests/test_core.py	100.0% <100.0%> (ø)
tests/test_filter.py	100.0% <100.0%> (ø)

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[danielhuppmann]

fyi @phackstock @gidden @coroa