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ffi.error: symbol 'setMsgSeverity' not found in library '<None>': /usr/bin/python3: undefined symbol: setMsgSeverity

See original GitHub issue

When trying to build 7.4.0 against Debian experimental, I got this error:

Running Sphinx v1.7.9

Configuration error:
There is a programable error in your configuration file:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/sphinx/config.py", line 161, in __init__
    execfile_(filename, config)
  File "/usr/lib/python3/dist-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "conf.py", line 81, in <module>
    from ocrmypdf import __version__ as OCRMYPDF_VERSION
  File "/<<PKGBUILDDIR>>/debian/.debhelper/ocrmypdf/__init__.py", line 36, in <module>
    from . import leptonica
  File "/<<PKGBUILDDIR>>/debian/.debhelper/ocrmypdf/leptonica.py", line 43, in <module>
    lept.setMsgSeverity(lept.L_SEVERITY_WARNING)
ffi.error: symbol 'setMsgSeverity' not found in library '<None>': /usr/bin/python3: undefined symbol: setMsgSeverity

The call to lept.setMsgSeverity is new. I tried patching that line into OCRmyPDF 7.2.1, which is the version currently in Debian experimental, and I get a slightly different error:

Running Sphinx v1.7.9

Configuration error:
There is a programable error in your configuration file:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/sphinx/config.py", line 161, in __init__
    execfile_(filename, config)
  File "/usr/lib/python3/dist-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "conf.py", line 81, in <module>
    from ocrmypdf import __version__ as OCRMYPDF_VERSION
  File "/<<PKGBUILDDIR>>/debian/.debhelper/ocrmypdf/__init__.py", line 36, in <module>
    from . import leptonica
  File "/<<PKGBUILDDIR>>/debian/.debhelper/ocrmypdf/leptonica.py", line 38, in <module>
    lept.setMsgSeverity(lept.L_SEVERITY_WARNING)
AttributeError: cffi library '<None>' has no function, constant or global variable named 'setMsgSeverity'

It would seem that the version of liblept5 in Debian does not export the setMsgSeverity function. So I was thinking of just patching it out. But I am worried about the <None>. It suggests that the ffi.dlopen call is not succeeding. What do you think?