Windows build for 2.0.0-dev.23 broken

[forman]

Windows build is currently broken, see first occurence in AppVeyor build history https://ci.appveyor.com/project/ccitools/cate-core/builds/20724879

cate\core\types.py:44: in <module>
    import geopandas
C:\Miniconda36-x64\envs\cate-env\lib\site-packages\geopandas\__init__.py:4: in <module>
    from geopandas.io.file import read_file
C:\Miniconda36-x64\envs\cate-env\lib\site-packages\geopandas\io\file.py:3: in <module>
    import fiona
C:\Miniconda36-x64\envs\cate-env\lib\site-packages\fiona\__init__.py:83: in <module>
    from fiona.collection import BytesCollection, Collection
C:\Miniconda36-x64\envs\cate-env\lib\site-packages\fiona\collection.py:9: in <module>
    from fiona.ogrext import Iterator, ItemsIterator, KeysIterator
E   ImportError: DLL load failed: The specified module could not be found.

Seems to be (again) a problem in the geopandas, fiona, gdal dependency chain as reported in so many places. See also Toblerity/Fiona#402.

[forman]

As a consequence, Cate now uses Python 3.7.

[forman]

see also conda-forge/geopandas-feedstock#46