Compat: pandas MultiIndex .labels to .codes

[J535D165]

Change pandas.MultiIndex.labels into pandas.MultiIndex.codes with backwards compatibility. This removes the future/deprecation warning

Closes #99

[codecov]

Codecov Report

Merging #103 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage    77.2%   77.27%   +0.07%     
==========================================
  Files          33       33              
  Lines        2338     2341       +3     
  Branches      376      376              
==========================================
+ Hits         1805     1809       +4     
+ Misses        408      407       -1     
  Partials      125      125

Impacted Files	Coverage Δ
recordlinkage/base.py	80.9% <100%> (+0.42%)	⬆️

---

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[codecov]

Codecov Report

Merging #103 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage    77.2%   77.27%   +0.07%     
==========================================
  Files          33       33              
  Lines        2338     2341       +3     
  Branches      376      376              
==========================================
+ Hits         1805     1809       +4     
+ Misses        408      407       -1     
  Partials      125      125

Impacted Files	Coverage Δ
recordlinkage/base.py	80.9% <100%> (+0.42%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 87a5f4a...b6beaf5. Read the comment docs.