NEP29: Require Python 3.7+, numpy 1.17+, declare 3.10 support

[effigies]

No description provided.

[codecov]

Codecov Report

Merging #1079 (d90e8a0) into master (4ecf756) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1079   +/-   ##
=======================================
  Coverage   92.32%   92.32%           
=======================================
  Files         100       99    -1     
  Lines       12216    12205   -11     
  Branches     2388     2388           
=======================================
- Hits        11278    11268   -10     
+ Misses        614      613    -1     
  Partials      324      324           

Impacted Files	Coverage Δ
nibabel/minc2.py	89.87% <100.00%> (ø)
nibabel/testing/__init__.py	100.00% <100.00%> (+0.94%)	⬆️

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