Eliminate file intermediary in translate algebra

[jclerman]

Summary of changes

Previously, rdflib.plugins.sparql.algebra.translateAlgebra() maintained state via a file, with a fixed filename query.txt. With this change, use of that file is eliminated; state is now maintained in memory (so that multiple concurrent translateAlgebra() calls, for example, should no longer interfere with each other).

The change is accomplished with no change to the client interface. Basically, the actual functionality has been moved into a class, which is instantiated and used as needed (once per call to algrebra.translateAlgebra()). The class is available for direct use, though that's not anticipated to be useful in normal operation.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• For changes that have a potential impact on users of this project:
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation (added PyDoc for new class)
  • Considered adding an example in ./examples for new features.
  • Considered updating our changelog (CHANGELOG.md).
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.805% (-0.002%) from 90.807% when pulling 9e06fb8 on jclerman:eliminate-file-intermediary-in-translateAlgebra into 4da67f9 on RDFLib:main.

[aucampia]

Thanks for the PR, quite important I would say, but will have to wait for the next release which hopefully will be not too distant in the future. I will do a thorough review next week.

[aucampia]

Thanks again for the PR, the change looks good, I only renamed AlgebraTranslator to _AlgebraTranslator so it is not part of our public API, and I made some small changes to the docstrings, mainly so that references to functions are picked up by Sphinx.

[aucampia]

I will merge this after releasing 6.3.2