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='2022-04-02 18:42:36 +0200'>2022-04-02</span></td><td><a href='/guix/commit/po/packages/fi.po?id=8a208df17fe81fbf34f5a86cc52bb0b8e33c7b96'>nls: Update translations.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/packages/fi.po: New file.
* po/packages/LINGUAS: Add it.
</span></span></td><td>Julien Lepiller</td></tr>