docs: configure a zero PYTHONHASHSEED value during the Sphinx documentation build

The CLI documentation for flask-limiter invokes a command within the library, flask limiter limits during rendering of the cli.rst reStructuredText file.

When the resulting library code runs, an iteration over an unordered Python set object containing string-valued HTTP methods occurs, and the results of this directly affect the output of the command, producing non-deterministic results.

To overcome this, we can configure a deterministic PYTHONHASHSEED environment variable so that Python's hashing of the stored objects within the collection becomes deterministic. A value of zero (0) appears to be used within a number of existing Debian source package debian/rules files, so we re-use that value here.

Closes #1064519.