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='2019-11-27 00:03:03 +0100'>2019-11-27</span></td><td><a href='/guix/commit/config-daemon.ac?id=7738a72186583afb3bb2e0a866c8aba130372400'>daemon: GC remove-unused-links phase uses 'statx' when available.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-11-13 23:26:35 +0100'>2019-11-13</span></td><td><a href='/guix/commit/config-daemon.ac?id=298fb2907e3f432cea7dee9f58e89ab8d9dbd56f'>daemon: Don't include &lt;linux/fs.h&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-08 11:49:24 +0200'>2019-09-08</span></td><td><a href='/guix/commit/config-daemon.ac?id=f6919ebdc6b0ce0286814cc6ab0564b1a4c67f5f'>daemon: Run 'guix substitute' directly and assume a single substituter.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-08 11:49:24 +0200'>2019-09-08</span></td><td><a href='/guix/commit/config-daemon.ac?id=bc69ea2d605810cc32e13ed03d5848b8dc358b61'>daemon: Run 'guix offload' directly.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-08 11:49:24 +0200'>2019-09-08</span></td><td><a href='/guix/commit/config-daemon.ac?id=9fcc35c51fd579b3bb7b4f61cba6973675c3922c'>daemon: Run 'guix perform-download' directly.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-08 11:49:24 +0200'>2019-09-08</span></td><td><a href='/guix/commit/config-daemon.ac?id=0c684b7edfacada4f576855dbb09291f04a84fa2'>daemon: Run 'guix authenticate' directly.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-08 11:49:24 +0200'>2019-09-08</span></td><td><a href='/guix/commit/config-daemon.ac?id=2e3e5d21988fc2cafb2a9eaf4b00976ea425629d'>daemon: Invoke 'guix gc --list-busy' instead of 'list-runtime-roots'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>