setup: embed Cython

Summary:
Use an `asset` to download Cython on demand. So we don't need to install Cython
as build dependency on all supported platforms, and maintain the "Cython"
package for those platforms.

Upgrade to the latest Cython by the way.

Reviewed By: singhsrb

Differential Revision: D13513514

fbshipit-source-id: 5ebe9a3e5b785a8f85cd51624663f9cc1e5c66fd

setup.py

@@ -8,7 +8,6 @@
 from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
-from distutils.version import LooseVersion
 import contextlib
 import ctypes
 import ctypes.util
@@ -97,18 +96,6 @@ else:
         return s
 
 
-try:
-    import Cython
-except ImportError:
-    havecython = False
-else:
-    havecython = LooseVersion(Cython.__version__) >= LooseVersion("0.22")
-
-if not havecython:
-    raise RuntimeError("Cython >= 0.22 is required")
-
-from Cython.Build import cythonize
-
 # Attempt to guide users to a modern pip - this means that 2.6 users
 # should have a chance of getting a 4.2 release, and when we ratchet
 # the version requirement forward again hopefully everyone will get
@@ -207,7 +194,7 @@ from distutils.spawn import spawn, find_executable
 from distutils import file_util
 from distutils.errors import CCompilerError, DistutilsError, DistutilsExecError
 from distutils.sysconfig import get_python_inc, get_config_var
-from distutils.version import StrictVersion, LooseVersion
+from distutils.version import StrictVersion
 from distutils_rust import RustExtension, RustBinary, RustVendoredCrates, BuildRustExt
 import distutils
 
@@ -564,12 +551,15 @@ class asset(object):
         self.version = version
 
     def ensureready(self):
-        """Download and extract the asset to self.destdir"""
+        """Download and extract the asset to self.destdir. Return full path of
+        the directory containing extracted files.
+        """
         if not self._isready():
             self._download()
             self._extract()
             self._markready()
         assert self._isready(), "%r should be ready now" % self
+        return pjoin(builddir, self.destdir)
 
     def _download(self):
         destpath = pjoin(builddir, self.name)
@@ -1768,6 +1758,20 @@ extmodules = [
     ),
 ]
 
+
+def cythonize(*args, **kwargs):
+    """Proxy to Cython.Build.cythonize. Download Cython on demand."""
+    cythonsrc = asset(
+        url="https://files.pythonhosted.org/packages/c1/f2/d1207fd0dfe5cb4dbb06a035eb127653821510d896ce952b5c66ca3dafa4/Cython-0.29.2.tar.gz"
+    )
+    path = cythonsrc.ensureready()
+    sys.path.insert(0, path)
+
+    from Cython.Build import cythonize
+
+    return cythonize(*args, **kwargs)
+
+
 # Cython modules
 # see http://cython.readthedocs.io/en/latest/src/reference/compilation.html
 cythonopts = {"unraisable_tracebacks": False, "c_string_type": "bytes"}