py3: implement __hash__ method in ContinuousMap

[egourgoulhon]

ContinuousMap defines its own __eq__ method, while inheritating from Morphism.__hash__. Now, as pointed out in #24551, Python3 breaks the inheritance of __hash__ when __eq__ is redefined. This ticket fixes this by implementing ContinousMap.__hash__. Moreover, contrary to Morphism.__hash__, the new method does not rely on repr() and guarantees that two continous maps that compare equal have the same hash.

Note that there is a proposal at #25393 to remove Morphism.__hash__.

ContinousMap (more precisely the derived class DiffMap) needs to be hashable because DiffMap objects serve as keys in the dictionary of vector field modules on a manifold, a module of vector fields on a differentiable manifold being essentially defined by a differentiable map (with the important special case of the identity map; see ​here for details).

[egourgoulhon]

New commits:

​83dcbb1

Add __hash__ to ContinousMap

[chapoton]

does seem to be a very rough hash... But it will do the job for the moment

[egourgoulhon]

Replying to chapoton:

does seem to be a very rough hash... But it will do the job for the moment

Yes it's pretty rough, but there should be no loss of efficiency, since continuous maps are only used as keys in the dictionary DifferentiableManifold._vector_field_modules, which is of very small size; most of the time, it has a single element (the continuous map being then merely the identity map).