Compile for python3 by default

Author: kngwyu

README.md

@@ -30,9 +30,9 @@ Supported python version
 -------------
 Currently 2.7, 3.5, 3.6, 3.7 are supported.
 
-By default, python version is automatically by execute `python`.
+By default, rust-numpy is built for Python3.
 
-You cal also specify python version manually, by adding a feature flag like
+If you want to compile for Python2, please specify
 
 ``` toml
 [dependencies.numpy]
@@ -41,7 +41,8 @@ features = ["python3"]
 ```
 .
 
-You can also specify python version in [setup.py](examples/simple-extension/setup.py).
+You can also automatically specify python version in [setup.py](examples/simple-extension/setup.py)
+, using [setuptools-rust](https://github.com/PyO3/setuptools-rust).
 
 
 Example

build.rs

@@ -16,11 +16,11 @@ use std::os::raw::c_void;
 fn get_numpy_api(module: &str, capsule: &str) -> *const *const c_void {
     let module = CString::new(module).unwrap();
     let capsule = CString::new(capsule).unwrap();
-    #[cfg(not(Py_3))]
+    #[cfg(feature = "python2")]
     unsafe fn get_capsule(capsule: *mut ffi::PyObject) -> *const *const c_void {
         ffi::PyCObject_AsVoidPtr(capsule) as *const *const c_void
     }
-    #[cfg(Py_3)]
+    #[cfg(not(feature = "python2"))]
     unsafe fn get_capsule(capsule: *mut ffi::PyObject) -> *const *const c_void {
         use std::ptr::null_mut;
         ffi::PyCapsule_GetPointer(capsule, null_mut()) as *const *const c_void