Deprecate setting default_file_mode, only allow setting to 'r'

[takluyver]

This is step 2 of removing the config option added to ease the transition from 2.x to 3.x:

1. Setting the default to anything other than 'r' (which is the default since 3.0) warns (done in Deprecate setting default_file_mode to values other than 'r' #1789 for h5py 3.2)
2. Setting the default to 'r' warns, and to anything else is an error (this PR)
3. Remove the default_file_mode option

We added the option (and indicated that the default would change) in 2.10 (released September 2019). The default changed to 'r' in 3.0 (October 2020). The warning for setting the default to modes other than 'r' was added in 3.2 (March 2021).

I'm provisionally marking this for 3.3, but I'm happy to leave it longer if people prefer. We could also start error-ing on modes other than r but leave the warning on setting the default to r for later.

[codecov]

Codecov Report

Merging #1839 (95ce39f) into master (c616b0f) will decrease coverage by 1.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1839      +/-   ##
==========================================
- Coverage   88.40%   87.27%   -1.14%     
==========================================
  Files          17       17              
  Lines        2285     2184     -101     
==========================================
- Hits         2020     1906     -114     
- Misses        265      278      +13     

Impacted Files	Coverage Δ
h5py/_hl/files.py	91.70% <100.00%> (+0.30%)	⬆️
h5py/_hl/filters.py	84.97% <0.00%> (-7.81%)	⬇️
h5py/_hl/attrs.py	95.20% <0.00%> (-0.99%)	⬇️
h5py/_hl/dims.py	95.18% <0.00%> (-0.70%)	⬇️
h5py/_hl/datatype.py	95.00% <0.00%> (-0.66%)	⬇️
h5py/_hl/selections.py	87.43% <0.00%> (-0.57%)	⬇️
h5py/_hl/base.py	95.37% <0.00%> (-0.27%)	⬇️
h5py/_hl/dataset.py	92.30% <0.00%> (-0.22%)	⬇️
h5py/_hl/group.py	96.62% <0.00%> (-0.13%)	⬇️
h5py/_hl/vds.py	97.33% <0.00%> (-0.04%)	⬇️
... and 1 more

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[takluyver]

I'm aiming for 3.3 fairly soon. On the one hand, 3 months is fairly short for going from a warning to an error. On the other hand, the proper fix - adding an explicit mode to h5py.File calls wherever necessary - is simple and obvious, and people have had the best part of 2 years to make that change (since 2.10 warned that the default was changing). So I will merge this in a few more days if no-one objects.

[tacaswell]

Given @aragilar' comment is #1909 (comment) I'm going to merge this.