Pyminc

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* support use of MINC_TOOLKIT environment variable to indicate libminc2.so location
(removing need to set LD_LIBRARY_PATH or similar)
* improve README

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* revert a commit (31) causing incorrect scaling of label files
* general code cleanup
* add Github actions:
* test on pull/pr to main/develop
* publish to PyPI on release
* drop support for unmaintained Python versions (3.5 and 3.6)

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* mincVolume.writeFile: don't choke on some numpy dtypes
* remove obsolete `weave` code from pyminc_test2.py

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* support non-C-contiguous ndarrays (37)
* fix a bug where a volume only written via hyperslab API would be deleted on `closeVolume` (32)
* remove old conditional code to support Python 2 (not supported since 0.53)
* substantial cleanup of the tests; new tests for non-contiguous arrays
* various minor code improvements

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* bugfix where valid range would be set incorrectly for label files (pyminc/pull/31)

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* solve some test flakiness by using a more appropriate assert (pyminc/issues/34)
- the exact mechanism causing the original flakiness is still unknown