bump required cython version

INSTALL.rst

@@ -271,7 +271,7 @@ The following software is highly recommended though not strictly required:
 
 In addition, to build a copy of Kwant that has been checked-out directly from
 `its Git repository <http://git.kwant-project.org/kwant>`_, you will also need
-`Cython <http://cython.org/>`_ 0.17.1 or newer.  You do not need Cython to build
+`Cython <http://cython.org/>`_ 0.22 or newer.  You do not need Cython to build
 Kwant that has been unpacked from a source .tar.gz-file.
 
 

setup.py

@@ -29,7 +29,7 @@ CONFIG_FILE = 'build.conf'
 README_FILE = 'README.rst'
 README_END_BEFORE = 'See also in this directory:'
 STATIC_VERSION_FILE = 'kwant/_static_version.py'
-REQUIRED_CYTHON_VERSION = (0, 17, 1)
+REQUIRED_CYTHON_VERSION = (0, 22)
 NO_CYTHON_OPTION = '--no-cython'
 TUT_DIR = 'tutorial'
 TUT_GLOB = 'doc/source/tutorial/*.py'