Geoplot

This release fixes some regressions in the unit tests. User-facing code is the same as in version `0.5.1`.

This maintenance release includes the follows breaking changes:

* The minimum Python version has been raised to 3.7, as Python 3.6 has now reached EOL (270).
* The handling of certain parameters (namely: `legend`, `cmap`, and `hue`) to `geoplot.kdeplot` has been changed. Previously these parameters were interpreted by `geoplot` directly. Now they are simply passed down to the underlying `seaborn.kdeplot` instance (271, 272).

This maintenance release includes the following non-breaking changes:
* Removed the deprecation warning for the long-removed `k` parameter (273).
* Fixed various deprecation notices in underlying libraries (268, 269).
* Miscellaneous minor fixes in code and docs.

This release pins `geopandas` to the minimum supported version. Due to `descartes` bit-rotting, we now rely on the `PolygonPatch` implementation internal to `geopandas>=0.9.0` (the most recent major version, at time of writing). See further https://github.com/geopandas/geopandas/pull/1677, #238, 249.

This maintenance release fixes a few bugs, updates the projection list (243), and removes this library's dependency on the no-longer-maintained descartes library ( 238).

Note: 0.4.2 was skipped due to a bug (245).

This is a maintenance release mostly fixing deprecations caused by changes in `geopandas` and `contextily`.

There is one minor user-facing change: the API for providing webmap tiles to `webmap` has changed. Refer to the documentation for details.

This release is a major breaking refactor of the colormap parameters in `geoplot`.

**Changes to the `scheme` parameter and the removal of `k` parameter**

In previous versions of `geoplot`, categorical colormaps were specified by a combination of the `scheme` and `k` string parameters. For example:

python
import geoplot as gplt
df = gpd.read_file(gplt.datasets.get_path('usa_cities'))
gplt.pointplot(df, hue='ELEV_IN_FT', cmap='viridis', scheme='EqualInterval', k=5)


In versions of `geoplot` 0.4.0 and later, the code to do this is now:

python
gplt.pointplot(df, hue='ELEV_IN_FT', cmap='viridis', scheme='EqualInterval')


Or this:

python
import mapclassify as mc
scheme = mc.EqualInterval(df['ELEV_IN_FT'], k=5)
gplt.pointplot(df, hue='ELEV_IN_FT', cmap='viridis', scheme=scheme)


This changeset was implemented primarily to make it possible to share the same colormap across plots. This was previously very difficult to do: see 163 and 182 for further context.

**Other changes**

This update also includes a number of minor bugfixes and docs changes.