Fix build with Boost + Python 3.7.

Taken from upstream:
https://github.com/ukoethe/vigra/commit/a6fa62663c6a6b752ed0707e95f643e25867a0f9

diff --git a/vigranumpy/src/core/vigranumpycore.cxx b/vigranumpy/src/core/vigranumpycore.cxx
index ec38d3636..c81c6ae52 100644
--- a/vigranumpy/src/core/vigranumpycore.cxx
+++ b/vigranumpy/src/core/vigranumpycore.cxx
@@ -61,7 +61,7 @@ UInt32 pychecksum(python::str const & s)
 	return checksum(data, size);
 #else
 	Py_ssize_t size = 0;
-	char * data = PyUnicode_AsUTF8AndSize(s.ptr(), &size);
+	const char * data = PyUnicode_AsUTF8AndSize(s.ptr(), &size);
 	return checksum(data, size);
 #endif
 }
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<div class='path'>path: <a href='/guix/log/?id=9209b772b583080178b469e9ae7f0d94f74ce50a'>root</a>/<a href='/guix/log/gnu?id=9209b772b583080178b469e9ae7f0d94f74ce50a'>gnu</a>/<a href='/guix/log/gnu/compression.scm?id=9209b772b583080178b469e9ae7f0d94f74ce50a'>compression.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/compression.scm?id=9209b772b583080178b469e9ae7f0d94f74ce50a&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-09-24 14:49:09 +0200'>2022-09-24</span></td><td><a href='/guix/commit/gnu/compression.scm?id=ebe9d660a55629f2506db124b0e016885fc61e5c'>gnu: Add compression module.</a><span class='msg-avail'>...<span class='msg-tooltip'>Move the compression record to a dedicated module so that it can be used
outside (guix scripts pack) module.

* guix/scripts/pack.scm (&lt;compressor&gt;, %compressors, lookup-compressor): Move
it to ...
* gnu/compression.scm: ... this new file.
* gnu/ci.scm: Adapt it.
* local.mk (GNU_SYSTEM_MODULES): Add it.
</span></span></td><td>Mathieu Othacehe</td></tr>