COMPAT: use mpl area legend if available

[sinhrks]

• closes area plots are causing my unit tests to fail #9161, closes Plot area legend rendered incorrectly when moved out of the plot #13544
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

Do not use area legend workaround if mpl >= 1.5.0 is available. Legends are now displayed properly if mpl >= 1.5.0.
#9161:

df = pd.DataFrame(np.random.rand(10, 3))
df.plot(kind='area', subplots=True, sharex=True, legend=True)

#13544:

df = pd.DataFrame(np.random.rand(20, 5), columns=['A', 'B', 'C', 'D', 'E'])
df.plot(kind='area', linewidth=0.1).legend(loc="center left", bbox_to_anchor=(1.02, 0.5))

[codecov-io]

Current coverage is 85.30% (diff: 72.72%)

Merging #13680 into master will decrease coverage by <.01%

@@             master     #13680   diff @@
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  Files           139        139          
  Lines         50143      50146     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42776      42776          
- Misses         7367       7370     +3   
  Partials          0          0          

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[jorisvandenbossche]

Looks good to me. @TomAugspurger Can you have a look?

[TomAugspurger]

Is this worth warning over? Seems similar to the subplots=True, axes=... case, which we do warn about.

[TomAugspurger]

Oh, I see below you were moving that from below. Never mind then.

[TomAugspurger]

Just the one question, but not really relevant to this PR. +1

Oh, might want a whatsnew entry.

[jreback]

@TomAugspurger status of this?