Allow specifying keywords as a list

[takluyver]

The keywords field in the metadata is a space-separated list, and flit expects this format straight from the user.

It might be nice to allow writing this as a proper list in pyproject.toml. It doesn't allow spaces in keywords, because the metadata format is still space-separated, but it's natural to expect a list to work.

[takluyver]

But it might need to be a comma separated list after all: https://mail.python.org/archives/list/distutils-sig@python.org/thread/LFTLGMIIB5PRG7NUUIGGGX4COGX53DMR/