Expect the pyo3 crate to be in scope for the codegen

This allows using generated code from crates that do not directly depend on pyo3.

E.g.:

```rust
extern crate my_pyo3_wrapper;

use my_pyo3_wrapper::pyo3;

#[py::modinit(rust2py)]
fn init_mod(py: Python, m: &PyModule) -> PyResult<()> {
    // ...
    Ok(())
}

pyo3-derive-backend/src/module.rs

@@ -44,7 +44,6 @@ pub fn py3_init(fnname: &syn::Ident, name: &String, doc: syn::Lit) -> Tokens {
         #[no_mangle]
         #[allow(non_snake_case, unused_imports)]
         pub unsafe extern "C" fn #cb_name() -> *mut ::pyo3::ffi::PyObject {
-            extern crate pyo3;
             use std;
             use pyo3::{IntoPyPointer, ObjectProtocol};
 
@@ -121,7 +120,6 @@ pub fn py2_init(fnname: &syn::Ident, name: &String, doc: syn::Lit) -> Tokens {
         #[no_mangle]
         #[allow(non_snake_case, unused_imports)]
         pub unsafe extern "C" fn #cb_name() {
-            extern crate pyo3;
             use std;
 
             // initialize python

pyo3-derive-backend/src/py_class.rs

@@ -41,7 +41,7 @@ pub fn build_py_class(ast: &mut syn::DeriveInput, attr: String) -> Tokens {
                 unused_qualifications, unused_variables, non_camel_case_types)]
         const #dummy_const: () = {
             use std;
-            extern crate pyo3 as _pyo3;
+            use pyo3 as _pyo3;
 
             #tokens
         };
@@ -372,7 +372,7 @@ fn impl_descriptors(cls: &syn::Ty, descriptors: Vec<(syn::Field, Vec<FnType>)>)
         #[allow(non_upper_case_globals, unused_attributes,
                 unused_qualifications, unused_variables, unused_imports)]
         const #dummy_const: () = {
-            extern crate pyo3 as _pyo3;
+            use pyo3 as _pyo3;
 
             #tokens
         };

pyo3-derive-backend/src/py_impl.rs

@@ -57,7 +57,7 @@ fn impl_methods(ty: &Box<syn::Ty>, impls: &mut Vec<syn::ImplItem>) -> Tokens {
         #[allow(non_upper_case_globals, unused_attributes,
                 unused_qualifications, unused_variables, unused_imports)]
         const #dummy_const: () = {
-            extern crate pyo3 as _pyo3;
+            use pyo3 as _pyo3;
 
             #tokens
         };

pyo3-derive-backend/src/py_proto.rs

@@ -128,7 +128,7 @@ fn impl_proto_impl(ty: &Box<syn::Ty>,
         #[allow(non_upper_case_globals, unused_attributes,
                 unused_qualifications, unused_variables)]
         const #dummy_const: () = {
-            extern crate pyo3 as _pyo3;
+            use pyo3 as _pyo3;
 
             #tokens