Fix warnings.

[jeffdav]

This fixes the following two warnings:

/Users/jeffdav/src/jeffdav-PythonKit/PythonKit/NumpyConversion.swift:140:34: warning: 'assign(from:count:)' is deprecated: renamed to 'update(from:count:)'
            buffPtr.baseAddress!.assign(from: ptr, count: scalarCount)
                                 ^
/Users/jeffdav/src/jeffdav-PythonKit/PythonKit/NumpyConversion.swift:140:34: note: use 'update(from:count:)' instead
            buffPtr.baseAddress!.assign(from: ptr, count: scalarCount)
                                 ^~~~~~
                                 update
/Users/jeffdav/src/jeffdav-PythonKit/Tests/PythonKitTests/PythonFunctionTests.swift:207:13: warning: initialization of immutable value 'testFunction' was never used; consider replacing with assignment to '_' or removing it
        let testFunction = PythonFunction { _ in
        ~~~~^~~~~~~~~~~~
        _

[jeffdav]

@compnerd Any concerns?