Retry HTTP 5xx errors

[jwodder]

Closes #93.

[codecov[bot]]

Codecov Report

Base: 90.97% // Head: 90.93% // Decreases project coverage by -0.04% :warning:

Coverage data is based on head (6bd1cac) compared to base (e2e4fc9). Patch coverage: 91.66% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #96 +/- ## ========================================== - Coverage 90.97% 90.93% -0.04% ========================================== Files 10 10 Lines 587 596 +9 ========================================== + Hits 534 542 +8 - Misses 53 54 +1 ``` | [Impacted Files](https://codecov.io/gh/datalad/datalad-fuse/pull/96?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/96?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=datalad#diff-ZGF0YWxhZF9mdXNlL2Zzc3BlYy5weQ==) | `83.62% <91.66%> (+0.29%)` | :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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[yarikoptic]

looks great to me, thanks! Unfortunately 1 simple import conflict came up. can't resolve on github interface. Please resolve to your liking.

[jwodder]

@yarikoptic Rebased.