Csr

This PR fixes a few problems to make the MKL interfaces more robust, particularly when working with very large matrices. All users should upgrade.

Per [NEP 29](https://numpy.org/neps/nep-0029-deprecation_policy.html), this release also drops support for Python 3.6 and NumPy 1.16.

Merged PRs

* Gate large-memory tests based on available memory (12) mdekstrand
* Support very large matrix multiplication with MKL (11) mdekstrand
* Enforce MKL interface limits (10) mdekstrand
* Further updates to version testing (9) mdekstrand
* Fix creation of large empty matrices (8) mdekstrand

0.3.0 was uploaded to PyPI with the wrong version.

This primarily adds the `pick_rows` method to streamline LensKit algorithms.

Merged PRs

* Add pick_rows method for row subsetting (6) mdekstrand
* Update Python versions (5) mdekstrand

This release changes the CSR class to use NumPy structrefs instead of jitclasses. This removes the need for different interfaces for compiled and pure code - all code can just use CSR, and Numba access the accelerated APIs.

This is a small tweak, fixes how the build script is packaged in the source distribution.

This is the first version of CSR.