Issue #437 Fix type hint to resolve Sphinx Extension error (sphinx_au…

…todoc_typehints)

Error:
Extension error (sphinx_autodoc_typehints):
Handler <function process_docstring at 0x7f5ae8e9c950> for event 'autodoc-process-docstring' threw an exception (exception: module 'matplotlib.colors' has no attribute 'ColorMap')

Cause:
Typehint was previously using different casing for `Colormap`.
Also, this type hint was encoded as a string before, but now we can use a regular type hint.

openeo/udf/xarraydatacube.py

@@ -156,18 +156,18 @@ def save_to_file(self, path: Union[str, Path], fmt=None, **kwargs):
             raise ValueError(fmt)
 
     def plot(
-            self,
-            title: str = None,
-            limits=None,
-            show_bandnames: bool = True,
-            show_dates: bool = True,
-            show_axeslabels: bool = False,
-            fontsize: float = 10.,
-            oversample: float = 1,
-            cmap: Union[str, 'matplotlib.colors.ColorMap'] = 'RdYlBu_r',
-            cbartext: str = None,
-            to_file: str = None,
-            to_show: bool = True
+        self,
+        title: str = None,
+        limits=None,
+        show_bandnames: bool = True,
+        show_dates: bool = True,
+        show_axeslabels: bool = False,
+        fontsize: float = 10.0,
+        oversample: float = 1,
+        cmap: Union[str, matplotlib.colors.Colormap] = "RdYlBu_r",
+        cbartext: str = None,
+        to_file: str = None,
+        to_show: bool = True,
     ):
         """
         Visualize a :py:class:`XarrayDataCube` with matplotlib