The current clang-format instructions are for Python2 Vim. If your Vim has only Python3 or if you load Python3 before Python2, those instructions won't work.
Therefore, (1) replace your clang-format.py with the file in this gist, from https://reviews.llvm.org/D23319. (2) Replace the relevant lines in .vimrc with:
map <C-K> :py3f ~/PATH/TO/clang-format.py<cr>
imap <C-K> <c-o>:py3f ~/PATH/TO/clang-format.py<cr>
N.B. Replace ~/PATH/TO
with the actual path to clang-format.py.