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='2022-06-23 12:52:16 +0100'>2022-06-23</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=c0afc06ecb5dd346aab753dfa6a3f5f819092ef5'>gnu: python-bandit: Update to 1.7.4.</a><span class='msg-avail'>...</span></td><td>jgart</td></tr>
<tr><td><span title='2022-06-21 17:30:44 +0200'>2022-06-21</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=8dbc8d480204df73dafb890c5695108a0a0cb811'>gnu: python-cliff: Update to 3.10.1.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-18 19:35:54 +0100'>2022-01-18</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=93006e55073dbe26f26fedbf63ec397cb01a4575'>gnu: Adjust merge resolution.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-17 11:47:09 -0500'>2022-01-17</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=276f40fdc349d2ad62582b23ea55e061b689cfc0'>Merge branch 'version-1.4.0'</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-01-10 11:44:44 -0500'>2022-01-10</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=0be48040dc30a07ee810d28d3c4011530910c502'>gnu: python-os-testr: Propagate python-testtools and python-babel.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-01-12 23:50:17 +0100'>2022-01-12</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=97db2f1302beb499f0387ddfe0eae45dc97e73d8'>gnu: python-oslo.log: Update to 4.6.1.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-12 23:50:17 +0100'>2022-01-12</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=4a6808226e1eb9796860aa9fa133e9c70c09e570'>gnu: python-oslo.serialization: Update to 4.2.0.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-12 23:50:17 +0100'>2022-01-12</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=850d2e8eef2c0ac9da52bbcbe13adb27260ca68d'>gnu: python-oslo.utils: Update to 4.12.0.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-12 23:50:16 +0100'>2022-01-12</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=a88cdb6b8fbe92a84f132e377fb31396134a4030'>gnu: python-oslo.config: Update to 8.7.1.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-11 20:35:00 +0100'>2022-01-11</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=92c873e38a9716d339fd7b0bcf37c6c2da6baf24'>gnu: python-os-testr: Add python-testrepository to inputs.</a><span class='msg-avail'>...</span></td><td>Tanguy Le Carrour</td></tr>
<tr><td><span title='2022-01-11 18:36:38 +0100'>2022-01-11</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=e768df37c19efbded660174fb12b3d4dc42c01e0'>gnu: python-oslo.i18n: Disable tests.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-11 18:36:38 +0100'>2022-01-11</span></td><td><a href='/guix/commit/gnu/packages/openstack.scm?id=0db89cc03f3cc64c29f325804815e990a237c05f'>gnu: python-os-testr: Update to 2.0.1.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>