Improve mypy config; remove typing casts

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (744de47) 75.90% compared to head (bedbc6e) 75.99%. Report is 12 commits behind head on draft.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## draft #369 +/- ## ========================================== + Coverage 75.90% 75.99% +0.09% ========================================== Files 14 14 Lines 2548 2558 +10 Branches 568 568 ========================================== + Hits 1934 1944 +10 Misses 451 451 Partials 163 163 ``` | [Files](https://app.codecov.io/gh/dandi/dandisets/pull/369?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [tools/backups2datalad/adandi.py](https://app.codecov.io/gh/dandi/dandisets/pull/369?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-dG9vbHMvYmFja3VwczJkYXRhbGFkL2FkYW5kaS5weQ==) | `81.49% <100.00%> (+0.67%)` | :arrow_up: | | [tools/backups2datalad/adataset.py](https://app.codecov.io/gh/dandi/dandisets/pull/369?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-dG9vbHMvYmFja3VwczJkYXRhbGFkL2FkYXRhc2V0LnB5) | `78.73% <100.00%> (+0.14%)` | :arrow_up: | | [tools/backups2datalad/zarr.py](https://app.codecov.io/gh/dandi/dandisets/pull/369?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-dG9vbHMvYmFja3VwczJkYXRhbGFkL3phcnIucHk=) | `73.55% <100.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.