0.4 release: speedups don't build with Python 3.7

[fkrull]

The generated speedups.c file included in the tinycss 0.4 release on PyPI doesn't build correctly on Python 3.7. The file was generated with an old version of Cython that didn't support Python 3.7.

Ref: cython/cython#1978
Ref: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=910751

[timokau]

For what its worth, the current workaround is to manually remove the speedups.c file (since otherwise cython won't regenerate it) and make sure cython is installed during build.

[fkrull]

Yeah, that's how we ended up solving it in the Debian package as well: https://salsa.debian.org/python-team/modules/python-tinycss/commit/632f2d4476721e3e8e606767b270cfd15b6c3c17