MAINT: Refactor Python engine empty line funcs

[gfyoung]

The Python engine's _empty and _check_empty methods were uninformative and undocumented.

This commit renames them to _is_line_empty and _remove_empty_lines respectively and provides appropriate documentation.

xref #15925 (comment)

[jreback]

lgtm. ping on green.

[codecov]

Codecov Report

Merging #15946 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #15946   +/-   ##
=======================================
  Coverage   90.98%   90.98%           
=======================================
  Files         145      145           
  Lines       49556    49556           
=======================================
  Hits        45091    45091           
  Misses       4465     4465

Flag	Coverage Δ
#multiple	88.75% <100%> (ø)	⬆️
#single	40.57% <28.57%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/io/parsers.py	95.64% <100%> (ø)	⬆️
pandas/core/internals.py	93.54% <0%> (-0.08%)	⬇️
pandas/util/testing.py	80.88% <0%> (+0.18%)	⬆️

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[gfyoung]

@jreback : All green and ready to go.

[jreback]

thanks!