Make AttributeManager.create consistent with __setitem__

[t20100]

This PR moves the call to base.guess_dtype from AttributeManager.__setitem__ to the beginning of AttributeManager.create so that both ways handle the dtype in a consistent way.

This closes #1057

[codecov]

Codecov Report

Merging #1265 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1265      +/-   ##
==========================================
+ Coverage   83.82%   83.83%   +0.01%     
==========================================
  Files          18       18              
  Lines        2170     2172       +2     
==========================================
+ Hits         1819     1821       +2     
  Misses        351      351

Impacted Files	Coverage Δ
h5py/_hl/attrs.py	86.4% <100%> (+0.22%)	⬆️

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[codecov]

Codecov Report

Merging #1265 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1265      +/-   ##
==========================================
+ Coverage   83.82%   83.83%   +0.01%     
==========================================
  Files          18       18              
  Lines        2170     2172       +2     
==========================================
+ Hits         1819     1821       +2     
  Misses        351      351

Impacted Files	Coverage Δ
h5py/_hl/attrs.py	86.4% <100%> (+0.22%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 596748d...088ae0d. Read the comment docs.

[scopatz]

This looks good to me. Can you please add something to the "whatisnew" file?

[t20100]

I updated whatsnew for 2.10.

[scopatz]

Thanks!