Adapt to python3.8 C API change (#1950)

* Adapt to python3.8 C API change

Do `Py_DECREF(type)` on all python objects on deallocation

fix #1946

* Add bare python3.8 build to CI matrix

While numpy/scipy wheels are available, run python3.8 test without them

.travis.yml

@@ -106,6 +106,33 @@ matrix:
           - lld-7
           - libc++-7-dev
           - libc++abi-7-dev  # Why is this necessary???
+  - os: linux
+    dist: xenial
+    env: PYTHON=3.8 CPP=17 GCC=7
+    name: Python 3.8, c++17, gcc 7 (w/o numpy/scipy) # TODO: update build name when the numpy/scipy wheels become available
+    addons:
+      apt:
+        sources:
+          - deadsnakes
+          - ubuntu-toolchain-r-test
+        packages:
+          - g++-7
+          - python3.8-dev
+          - python3.8-venv
+    # Currently there is no numpy/scipy wheels available for python3.8
+    # TODO: remove next before_install, install and script clause when the wheels become available
+    before_install:
+      - pyenv global $(pyenv whence 2to3)  # activate all python versions
+      - PY_CMD=python3
+      - $PY_CMD -m pip install --user --upgrade pip wheel setuptools
+    install:
+      - $PY_CMD -m pip install --user --upgrade pytest
+    script:
+      - |
+        # Barebones build
+        cmake -DCMAKE_BUILD_TYPE=Debug -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -DPYTHON_EXECUTABLE=$(which $PY_CMD) .
+        make pytest -j 2
+        make cpptest -j 2
   - os: osx
     name: Python 2.7, c++14, AppleClang 7.3, CMake test
     osx_image: xcode7.3

include/pybind11/detail/class.h

@@ -350,6 +350,7 @@ extern "C" inline void pybind11_object_dealloc(PyObject *self) {
     auto type = Py_TYPE(self);
     type->tp_free(self);
 
+#if PY_VERSION_HEX < 0x03080000
     // `type->tp_dealloc != pybind11_object_dealloc` means that we're being called
     // as part of a derived type's dealloc, in which case we're not allowed to decref
     // the type here. For cross-module compatibility, we shouldn't compare directly
@@ -357,6 +358,11 @@ extern "C" inline void pybind11_object_dealloc(PyObject *self) {
     auto pybind11_object_type = (PyTypeObject *) get_internals().instance_base;
     if (type->tp_dealloc == pybind11_object_type->tp_dealloc)
         Py_DECREF(type);
+#else
+    // This was not needed before Python 3.8 (Python issue 35810)
+    // https://github.com/pybind/pybind11/issues/1946
+    Py_DECREF(type);
+#endif
 }
 
 /** Create the type which can be used as a common base for all classes.  This is