PythonConvert should produce and expect plain integers as representation of TreeValueInt32 to avoid truncation issues

[drjrm]

Originally a Python integer input to PythonConvert was expected to have the Python long type (unlimited and variable precision). This could lead to wrong expectations that it would store at unlimited precision, whereas it will always truncate to 32-bit.

We suggest:

• When converting from a tree to Python, PythonConvert always produces Python plain integers as representation of 32-bit integers stored on the tree
• When converting from a tree to Python any plain integers are accepted and converted to TreeInt32
• When converting from a tree to Python long integers are only accepted if they will fit in a 32-bit representation, and then stored as TreeInt32

[drjrm]

Implemented in d5e87b1fd9db05f8264e88ce1a858c9ca8ebeb19.