src/bin/sage-cython: new cython wrapper with the venv sys.path

When Sage is installed in a venv, sagelib gets added to sys.path by
virtue of venv/bin/python, which basically runs whatever python you
were going to run anyway, but with a modified sys.path.

If however you are building an external cython package using the
system cython, this falls apart: the system cython is run under the
system python, which does not have sagelib in its sys.path. This makes
it impossible to build cython SPKGs that depend on sagelib.

To address the issue, we add a tiny wrapper called sage-venv-cython
that essentially runs "python /path/to/cython" rather than"cython".
To use it, simply set

  export CYTHON=sage-cython

in spkg-install.in. (By the time the package is built, the wrapper will
be in $PATH.)

src/bin/sage-cython

@@ -0,0 +1,23 @@
+#!/usr/bin/env sage-python
+#
+# A wrapper that calls "python /path/to/cython" rather than executing
+# "cython" directly. This ensures that, within a venv, cython is
+# executed using the venv python and not the system python. The
+# distinction is important because cython uses the same sys.path as
+# the python it's running under; if we use the system python to run
+# cython, then cython won't be able to see sage's own pxd files.
+#
+# This only matters when building external cython packages that
+# make use of sage's own C extensions.
+
+import os
+import shutil
+import sys
+
+python = sys.executable
+cython = shutil.which("cython")
+
+args = sys.argv[1:]
+args.insert(0, python)
+args.insert(1, cython)
+os.execv(python, args)