add an example for `pysvn`'s external dependencies

[rgommers]

See https://discuss.python.org/t/pep-725-specifying-external-dependencies-in-pyproject-toml/31888/19. There's a large set of dependencies, and they vary across platforms. Construct the [external] table for it and see if we can express everything that's needed there and if the end result is readable.

[rgommers]

Last comment on this topic:

That’s a complicated set of dependencies. It seems like a nice test case actually. I have opened add an example for pysvn's external dependencies · Issue #9 · rgommers/peps · GitHub to work on an example of what the [external] table for pysvn should contain. I could give that a try later this week. Maybe you can comment on that issue with some more context? For example, there doesn’t seem to be a release on PyPI nor a pyproject.toml file, so I’m not sure if it’s helpful to try this.

Listed dependencies:

For fedora my pysvn deps are:

Requires
apr-devel
gcc-c++
glibc-langpack-en
krb5-devel
make
neon-devel
openssl-devel
python3-devel
python3-pycxx-devel >= 7.1.8
subversion
subversion-devel

On windows I have to compile from source the following to allow pysvn to be built.

build zlib
build openssl
build apr
build serf
build svn

[rgommers]

Okay, I looked at it, there's nothing that is a hard blocker in principle, but this is too much work and pretty much worst-case in terms of how it fits into the existing structure of this repo and maintenance / best practices:

• It isn't on PyPI, but on SourceForge: https://pysvn.sourceforge.io/. So nowhere to point at really for obtaining an sdist
• There's actually another pysvn on PyPI that is name-squatting (unclear if it's malicious): https://pypi.org/project/pysvn/
• PySvn depends on PyCXX which is also SourceForge-only (under the name CXX there) with no nice way to have a PURL (and pkg:generic/cxx would be quite bad): https://cxx.sourceforge.net/

Since all the dependencies are available in Fedora this should be doable, but it doesn't make sense without it being on PyPI.