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[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://codecov.io/gh/nipy/heudiconv/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy) | Coverage Δ | | |---|---|---| | [heudiconv/heuristics/multires\_7Tbold.py](https://codecov.io/gh/nipy/heudiconv/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L2hldXJpc3RpY3MvbXVsdGlyZXNfN1Rib2xkLnB5) | `14.28% <0.00%> (ø)` | | | [heudiconv/info.py](https://codecov.io/gh/nipy/heudiconv/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L2luZm8ucHk=) | `100.00% <ø> (ø)` | | | [heudiconv/bids.py](https://codecov.io/gh/nipy/heudiconv/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L2JpZHMucHk=) | `90.51% <100.00%> (ø)` | | | [heudiconv/dicoms.py](https://codecov.io/gh/nipy/heudiconv/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L2RpY29tcy5weQ==) | `86.20% <100.00%> (ø)` | | | [heudiconv/tests/test\_dicoms.py](https://codecov.io/gh/nipy/heudiconv/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L3Rlc3RzL3Rlc3RfZGljb21zLnB5) | `85.88% <100.00%> (ø)` | | | [heudiconv/tests/test\_regression.py](https://codecov.io/gh/nipy/heudiconv/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L3Rlc3RzL3Rlc3RfcmVncmVzc2lvbi5weQ==) | `93.25% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy)

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[yarikoptic]

Thank you!

[github-actions[bot]]

:rocket: PR was released in v0.13.0 :rocket: