SUPP: Remove pandas deprecation warnings

[toryhaavik]

Closes #1946

[codecov]

Codecov Report

Merging #1950 into master will decrease coverage by 3.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1950      +/-   ##
==========================================
- Coverage   87.67%   84.39%   -3.28%     
==========================================
  Files          93       93              
  Lines       16968    16968              
  Branches     2144     2144              
==========================================
- Hits        14876    14320     -556     
- Misses       1683     2237     +554     
- Partials      409      411       +2

Impacted Files	Coverage Δ
ibis/pandas/execution/generic.py	85.01% <100%> (-0.45%)	⬇️
ibis/sql/postgres/udf/api.py	32.78% <0%> (-57.38%)	⬇️
ibis/bigquery/client.py	41.1% <0%> (-53.39%)	⬇️
ibis/common/geospatial.py	31.39% <0%> (-50%)	⬇️
ibis/bigquery/compiler.py	59.92% <0%> (-37.5%)	⬇️
ibis/bigquery/udf/api.py	80.48% <0%> (-14.64%)	⬇️
ibis/expr/api.py	82.96% <0%> (-9.74%)	⬇️
ibis/omniscidb/client.py	52.5% <0%> (-8.06%)	⬇️
ibis/sql/alchemy.py	87.31% <0%> (-5.21%)	⬇️
ibis/impala/compiler.py	91.23% <0%> (-5.2%)	⬇️
... and 9 more

[xmnlab]

LGTM! thanks