Why is pip installing "Twisted[tls]>16.4" when Tahoe-LAFS asks for
"Twisted[tls,conch] >= 18.4.0"?

Compute >= 2 GiB share size correctly

Fix test coverage collection and reporting from the Nix CI job

Also document some of the other decisions in the .coveragerc file

The paths are even weirder inside the build environment.

We're abusing combine to rewrite paths in the coverage data.  combine isn't
intended to be used with only one coverage data file named ".coverage".  So
enable parallel mode so we have only one coverage data file with the naming
scheme "combine" expects.

Maybe this gets us working path rewriting on Windows,

codecov docs say that xml is supported and explicitly call out Python
".coverage" format as unsupported.

https://docs.codecov.io/docs/supported-report-formats

Something fails with "No usable data files" on CI, not sure what

Disable the Eliot test suite in the Nix package build