Fix metrics.run to work with pandas >= 1.1.0

[Lowess]

@ancadumitrache This PR is a small fix to solve some issues we ran into while using pandas >= 1.1.0 it's been tested with pandas == 1.2.4 and we got the same results as before.

Please consider this PR for a main stream merge and release !

Thanks

[codecov-io]

Codecov Report

Merging #5 (747c1c8) into master (f1a494b) will increase coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   92.04%   92.11%   +0.07%     
==========================================
  Files           8        9       +1     
  Lines         553      558       +5     
==========================================
+ Hits          509      514       +5     
  Misses         44       44              

Impacted Files	Coverage Δ
crowdtruth/models/metrics.py	99.26% <100.00%> (+<0.01%)	:arrow_up:
crowdtruth/__init__.py	100.00% <0.00%> (ø)

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[Lowess]

@oana-inel @Bouncer FYI

[cyriltw]

+1 on this PR. I had to apply this change to get it working.