Gradle requirements task does not depend on Python version

[mhsmith]

It depends on it indirectly by depending on buildPackagesTask, but if the requested Python minor version isn't installed, that task may use the same buildPython for multiple Python versions, and then the requirements task will not be rerun when the version changes between them.

The simplest solution may be to make it depend on only two things: the buildPython and the pip_install command line.

See if this affects any other tasks.