Add compatability with Bazel 0.26

Make co_argcount for positional-only arguments behave correctly, now that CPython 3.8 (beta 1) includes the pos-only count in it rather than adding it.

Avoid checking Py_TPFLAGS_HAVE_FINALIZE in Py3.8 and later since CPython now relies on the feature being there.

Fixes GH-2941.

Closes GH-2976.

When passing keyword arguments from unpacked dict literals, prevent duplicate dict keys from raising an error and instead overwrite the previous one.
Closes GH-2963.

This was broken by the attempt to exclude C++ operations from true division.

Move the variable declarations in a C helper function to where they are actually used to avoid "unused" warnings in Py3.

This was broken by the attempt to exclude C++ operations from true division.

This fixes Github issue #1950.

This fixes Github issue #1950.

Fix compile error with 3.8