Fix syntax error for Python < 3.8

[fepegar]

Description
The lazy-loading features added recently broke the CI for Python<3.8. This commit makes the code compatible with older Python versions.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed
• If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[codecov]

Codecov Report

Merging #639 (9eb738a) into master (cf656a7) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #639      +/-   ##
==========================================
+ Coverage   96.83%   96.85%   +0.01%     
==========================================
  Files         126      126              
  Lines        6516     6517       +1     
==========================================
+ Hits         6310     6312       +2     
+ Misses        206      205       -1     

Impacted Files	Coverage Δ
torchio/data/io.py	96.42% <100.00%> (ø)
...o/transforms/augmentation/intensity/random_swap.py	97.02% <0.00%> (+1.02%)	⬆️

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