Numpi

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- MAINT: Switched build system to `flit`

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- MAINT: Made `scipy` dependency optional

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- ENH: Support for masked arrays

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- ENH: Reduction for computing arithmetic mean

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- Drop support for python3.6 and take python3.9 in testing
- Fix bug in CI installation

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- ENH: Optimization: parallelized the bound constrained conjugate gradient without restart
- ENH: Optimization: implement bound constrained conjugate gradient without restart when the constrained set changes
- ENH: Optimization: implement bound constrained conjugate gradient with restart when the constrained set changes