Remove use of `six`

[jwodder]

heudiconv requires Python 3.7 or higher, so there is no need to use six.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 :warning:

Comparison is base (b2e5d03) 81.30% compared to head (b76d432) 81.29%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #655 +/- ## ========================================== - Coverage 81.30% 81.29% -0.01% ========================================== Files 42 42 Lines 3888 3887 -1 ========================================== - Hits 3161 3160 -1 Misses 727 727 ``` | [Impacted Files](https://codecov.io/gh/nipy/heudiconv/pull/655?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy) | Coverage Δ | | |---|---|---| | [heudiconv/info.py](https://codecov.io/gh/nipy/heudiconv/pull/655?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/tests/test\_tarballs.py](https://codecov.io/gh/nipy/heudiconv/pull/655?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L3Rlc3RzL3Rlc3RfdGFyYmFsbHMucHk=) | `100.00% <ø> (ø)` | | | [heudiconv/tests/test\_heuristics.py](https://codecov.io/gh/nipy/heudiconv/pull/655?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L3Rlc3RzL3Rlc3RfaGV1cmlzdGljcy5weQ==) | `100.00% <100.00%> (ø)` | | | [heudiconv/tests/test\_main.py](https://codecov.io/gh/nipy/heudiconv/pull/655?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-aGV1ZGljb252L3Rlc3RzL3Rlc3RfbWFpbi5weQ==) | `100.00% <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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[github-actions[bot]]

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