Add Python 3.11 support (#3231)

Even though Python 3.11 is at the development stage now, there are several breaking changes that we want to address. Namely,

* Replace "Py_TYPE(obj) = type" with:
  "Py_SET_TYPE(obj, type)"
* Replace "Py_REFCNT(Py_None) += n" with:
  "Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + n)"
* Add pythoncapi_compat.h to get Py_SET_TYPE() and Py_SET_REFCNT() on
  Python 3.9 and older. File copied from:
  https://github.com/pythoncapi/pythoncapi_compat

On Python 3.10, Py_REFCNT() can no longer be used to set a reference
count:

* https://docs.python.org/dev/c-api/structures.html#c.Py_REFCNT
* https://docs.python.org/dev/whatsnew/3.10.html#id2

On Python 3.11, Py_TYPE() can no longer be used to set an object
type:

* https://docs.python.org/dev/c-api/structures.html#c.Py_TYPE
* https://docs.python.org/dev/whatsnew/3.11.html#id2

src/core/buffer.cc

@@ -24,6 +24,7 @@
 #include <cstring>             // std::strerror, std::memcpy
 #include <mutex>               // std::mutex, std::lock_guard
 #include "buffer.h"
+#include "lib/pythoncapi_compat.h" // Py_SET_REFCNT()
 #include "mmm.h"                  // MemoryMapWorker, MemoryMapManager
 #include "python/pybuffer.h"      // py::buffer
 #include "utils/alloc.h"          // dt::malloc, dt::realloc
@@ -235,7 +236,7 @@ class BufferImpl
         PyObject** elements = static_cast<PyObject**>(data_);
         for (size_t i = 0; i < n; ++i) {
           XAssert(elements[i] != nullptr);
-          XAssert(elements[i]->ob_refcnt > 0);
+          XAssert(Py_REFCNT(elements[i]) > 0);
         }
       }
     }
@@ -929,7 +930,7 @@ class Mmap_BufferImpl : public BufferImpl, MemoryMapWorker {
       for (size_t i = 0; i < n; ++i) {
         data[i] = Py_None;
       }
-      Py_None->ob_refcnt += n;
+      Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + n);
     }
     impl_->contains_pyobjects_ = true;
     return *this;
@@ -954,7 +955,7 @@ class Mmap_BufferImpl : public BufferImpl, MemoryMapWorker {
           if (n_new > n_old) {
             PyObject** data = static_cast<PyObject**>(xptr());
             for (size_t i = n_old; i < n_new; ++i) data[i] = Py_None;
-            Py_None->ob_refcnt += n_new - n_old;
+            Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + n_new - n_old);
           }
         } else {
           impl_->resize(newsize);
@@ -1018,7 +1019,7 @@ class Mmap_BufferImpl : public BufferImpl, MemoryMapWorker {
         size_t i = 0;
         for (; i < n_copy; ++i) Py_INCREF(newdata[i]);
         for (; i < n_new; ++i) newdata[i] = Py_None;
-        Py_None->ob_refcnt += n_new - n_copy;
+        Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + n_new - n_copy);
       }
       impl_->release();  // noexcept
     }

src/core/column/const_na.cc

@@ -19,6 +19,7 @@
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 // IN THE SOFTWARE.
 //------------------------------------------------------------------------------
+#include "lib/pythoncapi_compat.h" // Py_SET_REFCNT()
 #include "column/const.h"
 #include "column/sentinel_fw.h"
 #include "column/sentinel_str.h"
@@ -84,7 +85,7 @@ static Column _fw_col(size_t nrows, SType stype) {
     });
 
   if (std::is_same<T, PyObject*>::value) {
-    Py_None->ob_refcnt += nrows;
+    Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + nrows);
     buf.set_pyobjects(/* clear_data= */ false);
   }
   return Column(new ColClass(nrows, stype, std::move(buf)));
@@ -101,7 +102,7 @@ static Column _special_col(size_t nrows) {
     });
 
   if (std::is_same<T, PyObject*>::value) {
-    Py_None->ob_refcnt += nrows;
+    Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + nrows);
     buf.set_pyobjects(/* clear_data= */ false);
   }
   return Column(new ColClass(nrows, std::move(buf)));