MAINT: update Cython imports

[tylerjereddy]

• use np.import_array() to prevent convoluted issues with
  newer versions of Cython when using cimport numpy .. in
  Cython sources

• in short, Cython and NumPy try to coordinate to do the right
  thing with interwoven header inclusions, but this explicit call
  prevents any confusion between the two

Fixes #3455

Changes made in this Pull Request:

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #3468 (9490241) into develop (46c8bad) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #3468   +/-   ##
========================================
  Coverage    93.75%   93.75%           
========================================
  Files          176      176           
  Lines        23163    23169    +6     
  Branches      3297     3297           
========================================
+ Hits         21717    21723    +6     
  Misses        1395     1395           
  Partials        51       51           

Impacted Files	Coverage Δ
package/MDAnalysis/lib/_augment.pyx	100.00% <100.00%> (ø)
package/MDAnalysis/lib/_cutil.pyx	100.00% <100.00%> (ø)
package/MDAnalysis/lib/c_distances.pyx	100.00% <100.00%> (ø)
package/MDAnalysis/lib/c_distances_openmp.pyx	100.00% <100.00%> (ø)
package/MDAnalysis/lib/formats/libdcd.pyx	88.53% <100.00%> (+0.04%)	⬆️
package/MDAnalysis/lib/qcprot.pyx	100.00% <100.00%> (ø)

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[richardjgowers]

LGTM