Clean implementation of the schedule callback

Originally reported by: Anselm Kruis (Bitbucket: akruis, GitHub: akruis)

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The current implementation of the schedule callback between the schedule decision and the actual switch is not clean and can cause problems. See issue #42 for details.

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• Bitbucket: https://bitbucket.org/stackless-dev/stackless/issue/48

Original comment by Anselm Kruis (Bitbucket: akruis, GitHub: akruis):

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In c5cac8a05d68 I documented the current limitations of the schedule callback.

Albeit the current implementation of the callback is not optimal and issue #42 proposes a good alternative, it is sufficient for its main purpose: to monitor tasklet scheduling. After we fixed #42 we got a good debugger support (Pydev).

Because our resources are limited and there is no pressing need, I close this issue.