Make PyType::name abi3 compatible

The implementation is more complex, because there's no equivalent to tp_name in the limited API

src/err.rs

@@ -481,7 +481,7 @@ impl<'a> std::fmt::Display for PyDowncastError<'a> {
             self.from
                 .repr()
                 .map(|s| s.to_string_lossy())
-                .unwrap_or_else(|_| self.from.get_type().name()),
+                .unwrap_or_else(|_| Cow::Borrowed(self.from.get_type().name())),
             self.to
         )
     }

src/types/typeobject.rs

@@ -6,8 +6,6 @@ use crate::err::{PyErr, PyResult};
 use crate::instance::PyNativeType;
 use crate::type_object::PyTypeObject;
 use crate::{ffi, AsPyPointer, PyAny, Python};
-use std::borrow::Cow;
-use std::ffi::CStr;
 
 /// Represents a reference to a Python `type object`.
 #[repr(transparent)]
@@ -39,8 +37,11 @@ impl PyType {
     }
 
     /// Gets the name of the `PyType`.
-    pub fn name(&self) -> Cow<str> {
-        unsafe { CStr::from_ptr((*self.as_type_ptr()).tp_name).to_string_lossy() }
+    pub fn name(&self) -> &str {
+        self.getattr("__qualname__")
+            .expect("Error accessing __qualname__")
+            .extract()
+            .expect("__qualname__ not a string")
     }
 
     /// Checks whether `self` is subclass of type `T`.