Basic type-checking

[jwodder]

Closes #94.

[jwodder]

Tests are currently failing because the Ubuntu APT mirror that the GitHub Actions environments use appears to be down.

[codecov[bot]]

Codecov Report

Base: 90.94% // Head: 90.97% // Increases project coverage by +0.03% :tada:

Coverage data is based on head (e95da5d) compared to base (56c2455). Patch coverage: 88.88% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #95 +/- ## ========================================== + Coverage 90.94% 90.97% +0.03% ========================================== Files 10 10 Lines 585 587 +2 ========================================== + Hits 532 534 +2 Misses 53 53 ``` | [Impacted Files](https://codecov.io/gh/datalad/datalad-fuse/pull/95?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad) | Coverage Δ | | |---|---|---| | [datalad\_fuse/fsspec.py](https://codecov.io/gh/datalad/datalad-fuse/pull/95?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9mdXNlL2Zzc3BlYy5weQ==) | `83.33% <88.88%> (+0.20%)` | :arrow_up: | 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=datalad). 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=datalad)

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[jwodder]

Tests are passing now.

[yarikoptic]

Thank you! Let's start with this.