Clarify valid key types

[cydanil]

Hi,
This PR aims to close #2233 , where it's suggested to clarify that only strings and bytes are valid key types.

Thanks!
Cyril

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.17 ⚠️

Comparison is base (2332968) 89.96% compared to head (971b1e9) 89.79%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2266      +/-   ##
==========================================
- Coverage   89.96%   89.79%   -0.17%     
==========================================
  Files          17       17              
  Lines        2382     2392      +10     
==========================================
+ Hits         2143     2148       +5     
- Misses        239      244       +5     

Impacted Files	Coverage Δ
h5py/_hl/base.py	95.95% <100.00%> (+0.03%)	⬆️

... and 2 files with indirect coverage changes

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[takluyver]

Thanks Cyril!