update cffi to v1.13.2

[mkroman]

This fixes some compilation issues for CPython 3.8+ that I was having while running make install

Running setup.py install for cffi ... error
ERROR: Command errored out with exit status 1:
 command: /home/mk/.pyenv/versions/3.8.1/bin/python3.8 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-b0snfbt_/cffi/setup.py'"'"'; __file__='"'"'/tmp/pip-install-b0snfbt_/cffi/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-sv6iwie5/install-record.txt --single-version-externally-managed --compile --install-headers /home/mk/.pyenv/versions/3.8.1/include/python3.8/cffi
     cwd: /tmp/pip-install-b0snfbt_/cffi/
Complete output (38 lines):
running install
running build
running build_py
creating build
creating build/lib.linux-x86_64-3.8
creating build/lib.linux-x86_64-3.8/cffi
copying cffi/recompiler.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/verifier.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/cffi_opcode.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/setuptools_ext.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/__init__.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/lock.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/api.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/vengine_gen.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/pkgconfig.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/ffiplatform.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/cparser.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/backend_ctypes.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/error.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/commontypes.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/model.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/vengine_cpy.py -> build/lib.linux-x86_64-3.8/cffi
copying cffi/_cffi_include.h -> build/lib.linux-x86_64-3.8/cffi
copying cffi/parse_c_type.h -> build/lib.linux-x86_64-3.8/cffi
copying cffi/_embedding.h -> build/lib.linux-x86_64-3.8/cffi
copying cffi/_cffi_errors.h -> build/lib.linux-x86_64-3.8/cffi
running build_ext
building '_cffi_backend' extension
creating build/temp.linux-x86_64-3.8
creating build/temp.linux-x86_64-3.8/c
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -DUSE__THREAD -DHAVE_SYNC_SYNCHRONIZE -I/usr/lib/libffi-3.2.1/include -I/home/mk/.pyenv/versions/3.8.1/include/python3.8 -c c/_cffi_backend.c -o build/temp.linux-x86_64-3.8/c/_cffi_backend.o
In file included from c/cffi1_module.c:20,
                 from c/_cffi_backend.c:7552:
c/call_python.c: In function ‘_get_interpstate_dict’:
c/call_python.c:20:30: error: dereferencing pointer to incomplete type ‘PyInterpreterState’ {aka ‘struct _is’}
   20 |     builtins = tstate->interp->builtins;
      |                              ^~
error: command 'gcc' failed with exit status 1
----------------------------------------

The relevant fixes are available in cffi v1.12.3 but I figured it'd be worth moving to v1.13 as I don't see any breaking changes.

I haven't looked at updates for any of the other dependencies as they install fine.

[mkroman]

There's a silent compatibility issue with CPython 3.8 as I described in #2769 - however, when testing with Python v3.7.6 and cffi v1.13.2 there doesn't seem to be any regression related to this issue.

[lyoshenka]

@mkroman thank you for this PR. We'll take a look as soon as we can. Can we show you some appreciation for the contribution?