DOC: update contextily example

[martinfleis]

Updated contextily example to use add_basemap from contextily 1.0. However, I would wait with merging until contextily 1.0.0 is stable as pip install contextily now installs 0.99.0 which does not work with this example.

Closes #1077

[jorisvandenbossche]

Yep, you're right about the pip issue. Another option could be to mention this explicitly in the beginning (and saying that pip users can upgrade with pip install --pre contextily)

[codecov]

Codecov Report

Merging #1088 into master will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
+ Coverage    90.2%   90.41%   +0.21%     
==========================================
  Files          19       20       +1     
  Lines        1837     1878      +41     
==========================================
+ Hits         1657     1698      +41     
  Misses        180      180

Impacted Files	Coverage Δ
geopandas/__init__.py	100% <0%> (ø)	⬆️
geopandas/tools/_show_versions.py	100% <0%> (ø)

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[martinfleis]

pip install --pre contextily will make sense only for some time (I assume you want to release stable soon?). I can put a note that the example requires contextily >= 1.0.0?

[jorisvandenbossche]

The original example of zoom=10 gave a less detailed background, with zoom=12 it actually takes some time ..

Do you think the higher zoom is better to illustrate? (if so, the longer doc build time is fine for me)

[martinfleis]

Yes. In reality, you'll rather want more detailed map than blurry one. I tried to simulate real-life use. I am aware that it takes longer, but I feel it makes more sense this way.