python-lz4 2.1 is broken, again

[totaamwin32]

on win32 clean install:

Traceback (most recent call last):
  File "E:\Xpra\trunk\src\dist/lib/xpra/net/compression.py", line 24, in <module>
    import lz4
  File "C:/msys64/mingw64/lib/python2.7/site-packages/lz4-2.1.0-py2.7-mingw.egg/lz4/__init__.py", line 8, in <module>
    VERSION = __version__
NameError: name '__version__' is not defined

[totaamwin32]

So, it turns out (unsurprisingly) that python-lz4 has changed how it gets its version information yet again - I can't remember if it's 3rd or 4th time now. And when it doesn't get its version number, the whole library fails to load!

For such a small and simple wrapper library, the number of releases and constant needless breakage is shocking.

Here's what it does now:

from pkg_resources import get_distribution, DistributionNotFound
try:
    __version__ = get_distribution(__name__).version
except DistributionNotFound:
    # package is not installed
    pass

And here's what I replaced it with to get on with more productive things:

    # package is not installed
    __version__ = "2.1.0"

Let's try to deal with this in #1883 so this won't break again next time there is an update.

[Antoine Martin]

r20171 fixes the packaging for macos by adding the patch to jhbuild.