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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.
Notes for packagers
===================
Mercurial ships a copy of the python-zstandard sources. This is used to
provide support for zstd compression and decompression functionality. The
module is not intended to be replaced by the plain python-zstandard nor
is it intended to use a system zstd library. Patches can result in hard
to diagnose errors and are explicitly discouraged as unsupported
configuration.