WIP: add mode(dropna=True)

[fulldecent]

• closes Feature request: DataFrame.mode(dropna=False) #17534
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @fulldecent! Thanks for submitting the PR.

• In the file pandas/core/algorithms.py, following are the PEP8 issues :

Line 671:1: E266 too many leading '#' for block comment

[jreback]

can you rebase and update

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c65a0f5). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master   #17547   +/-   ##
=========================================
  Coverage          ?    91.2%           
=========================================
  Files             ?      163           
  Lines             ?    49606           
  Branches          ?        0           
=========================================
  Hits              ?    45245           
  Misses            ?     4361           
  Partials          ?        0

Flag	Coverage Δ
#multiple	88.99% <100%> (?)
#single	40.19% <75%> (?)

Impacted Files	Coverage Δ
pandas/core/series.py	94.92% <100%> (ø)
pandas/core/frame.py	97.77% <100%> (ø)
pandas/core/algorithms.py	94.14% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c65a0f5). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master   #17547   +/-   ##
=========================================
  Coverage          ?    91.2%           
=========================================
  Files             ?      163           
  Lines             ?    49606           
  Branches          ?        0           
=========================================
  Hits              ?    45245           
  Misses            ?     4361           
  Partials          ?        0

Flag	Coverage Δ
#multiple	88.99% <100%> (?)
#single	40.19% <75%> (?)

Impacted Files	Coverage Δ
pandas/core/algorithms.py	94.14% <100%> (ø)
pandas/core/series.py	94.92% <100%> (ø)
pandas/core/frame.py	97.77% <100%> (ø)

---

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 c65a0f5...87972a5. Read the comment docs.

[fulldecent]

@jreback Updated and rebased.

[fulldecent]

Not sure I can make it work, but I found where this should go.

[jreback]

closing as stale, if you want to work on this, pls ping.