error: Accessing "__init__" on an instance is unsound, since instance.__init__ could be from an incompatible subclass

I added a couple of type ignores in #227, but this should probably be fixed:
```
odc/geo/crs.py:126: error: Accessing "__init__" on an instance is unsound, since instance.__init__ could be from an incompatible subclass  [misc]
odc/geo/geom.py:940: error: Accessing "__init__" on an instance is unsound, since instance.__init__ could be from an incompatible subclass  [misc]
```