Fix wheel generation on macOS Big Sur, switch to macos-bigsur image…

… on AppVeyor (#3174)

- add a workaround to generate proper wheel names on macOS Big Sur. We should still properly deprecate the `distutils` module at some point;
- switch to the most recent macOS image on AppVeyor.

Closes #3175

ci/appveyor.yml

@@ -1,7 +1,7 @@
 image:
   - Visual Studio 2019
   - Ubuntu
-  - macOS
+  - macos-bigsur
 
 init:
   # Uncomment the line below to enable RDP access

ci/xbuild/wheel.py

@@ -449,8 +449,13 @@ def get_tag(self):
         if self._tag is None:
             impl_tag = self._get_python_tag()
             abi_tag = self._get_abi_tag()
-            plat_tag = distutils.util.get_platform() \
-                       .replace('.', '_').replace('-', '_')
+            # TODO: remove `distutils` dependency as it has been deprecated
+            platform = distutils.util.get_platform()
+            # Meanwhile, a workaround for macOS Big Sur, see #3175
+            if platform.startswith("macosx-11"):
+                plat_tag = re.sub("-11(.*)-", "_11_0_", platform)
+            else:
+                plat_tag = platform.replace('.', '_').replace('-', '_')
             self._tag = "%s-%s-%s" % (impl_tag, abi_tag, plat_tag)
         return self._tag
 

src/core/column/categorical.cc

@@ -69,7 +69,7 @@ size_t Categorical_ColumnImpl<T>::n_children() const noexcept {
 
 template <typename T>
 const Column& Categorical_ColumnImpl<T>::child(size_t i) const {
-  xassert(i == 0);
+  xassert(i == 0); (void) i;
   return categories_;
 }