Allow GSHHS levels 5 and 6 for Antarctica.

The documentation for GSHHSFeature already states that levels 1 through
6 are allowed but the code so far only allowed 1 through 4.

As per https://www.ngdc.noaa.gov/mgg/shorelines/data/gshhg/readme.txt
levels 5 and 6 were introduced in GSHHS version 2.3.0 in February 2014
to distinguish the Antarctic ice-front (L5) and grounding lines (L6).

Importantly, this also means that L1 does not contain any coastlines for
Antarctica, so allowing only L1-L4 is insufficient.

See also https://www.soest.hawaii.edu/pwessel/gshhg/.

Author: kdungs

lib/cartopy/feature/__init__.py

@@ -365,7 +365,7 @@ def __init__(self, scale='auto', levels=None, **kwargs):
         if levels is None:
             levels = [1]
         self._levels = set(levels)
-        unknown_levels = self._levels.difference([1, 2, 3, 4])
+        unknown_levels = self._levels.difference([1, 2, 3, 4, 5, 6])
         if unknown_levels:
             raise ValueError(f"Unknown GSHHS levels {unknown_levels!r}.")