assign a filler to __version__ if it cannot be imported

[bendichter]

Motivation

fix #905

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (19c56f6) 88.32% compared to head (9621bf6) 88.32%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #907   +/-   ##
=======================================
  Coverage   88.32%   88.32%           
=======================================
  Files          45       45           
  Lines        9274     9274           
  Branches     2649     2649           
=======================================
  Hits         8191     8191           
  Misses        765      765           
  Partials      318      318           

Impacted Files	Coverage Δ
src/hdmf/__init__.py	61.90% <ø> (ø)

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[rly]

This works locally for me. MWE:

conda create --name test python=3.9 --yes
conda activate test
git checkout 3.6.0
pip install -e .
git checkout fix/905
python -c "import hdmf; print(hdmf.__version__)"