Remove kludgy support for older versions of pydicom and dcmstack

[jwodder]

By requiring pydicom >= 1.0.0 and dcmstack >= 0.7.0 (the latter of which we already do a stronger version of), we no longer need any kludgy workarounds to support older versions.

[codecov]

Codecov Report

Patch coverage: 83.33% and project coverage change: +0.24 🎉

Comparison is base (012895d) 81.22% compared to head (ff605d8) 81.47%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #662      +/-   ##
==========================================
+ Coverage   81.22%   81.47%   +0.24%     
==========================================
  Files          43       41       -2     
  Lines        3925     3902      -23     
==========================================
- Hits         3188     3179       -9     
+ Misses        737      723      -14     

Impacted Files	Coverage Δ
heudiconv/heuristics/multires_7Tbold.py	14.28% <0.00%> (ø)
heudiconv/info.py	100.00% <ø> (ø)
heudiconv/bids.py	90.51% <100.00%> (ø)
heudiconv/dicoms.py	86.20% <100.00%> (ø)
heudiconv/tests/test_dicoms.py	85.88% <100.00%> (ø)
heudiconv/tests/test_regression.py	93.25% <100.00%> (ø)

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[yarikoptic]

Thank you!

[github-actions]

🚀 PR was released in v0.13.0 🚀