Disallow Cloning() and Override()

[bdbaddog]

This is added as a solution to the issue described in PR #4041.

Basically Override()'s are meant to be a light weight deadending copy of an Environment() typically created automatically by SCons when an env var is specialized in a Builder() call.

Here's a possible way to disallow this, from @mwichmann (from: #4041 (comment))

    def __copy__(self):
        from copy import Error

        raise Error("un(shallow)copyable object of type %s" % type(self))