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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<tr><td><span title='2023-07-18 00:41:15 +0200'>2023-07-18</span></td><td><a href='/guix/commit/doc/environment-gdb.scm?id=602e32c8e0e36d149e1424d8023a214abd3d34ba'>doc: Update GDB "guix shell" example to use autoconf-2.69</a><span class='msg-avail'>...</span></td><td>fanquake</td></tr>
<tr><td><span title='2022-01-14 22:39:24 +0100'>2022-01-14</span></td><td><a href='/guix/commit/doc/environment-gdb.scm?id=b0aab0f255556c8c5027e3ad11bfc7826bb1f609'>doc: Update "guix shell -D -f" example to new input style.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>