doc: use an absolute path in sys.path to work around a python DLL loading bug This bug[1] was causing the zstd entry under "Available Compression Engines" to be omitted from the documentation for some versions of python3. I could upgrade, but past upgrades have broken venvs and clobbered installed packages. It's a trivial workaround, so there's no sense in leaving this subtle issue. It was flushed out by changing the module policy here from 'allow' to 'c', and seeing this error: ImportError: DLL load failed while importing parsers: The parameter is incorrect. [1] https://github.com/python/cpython/issues/87271 Differential Revision: https://phab.mercurial-scm.org/D12599