remove fsspec or s3fs from upstream testing

[d-v-b]

s3fs depends on an exact version of fsspec, so you cannot install the dev version of s3fs in the same environment as the dev version of fsspec while respecting that versioning requirement. See a conversation about that here: fsspec/s3fs#966.

We could:

• hack around this with an upstream testing script that defies the declared requirements of s3fs and installs dev fsspec
• remove s3fs from our upstream testing, and just use the dev version of fsspec
• remove the explicit fsspec dep from our upstream tests, and instead use the version that s3fs needs
• have two upstream test environments, one with dev fsspec, the other with dev s3fs.

I forked s3fs and changed the fsspec requirement, but that's going to be a chore to maintain, so this is not a real long-term solution. curious to hear other ideas.

[d-v-b]

important addendum: the incompatibility between the dev version of fsspec and the dev version of s3fs is breaking our upstream tests.