Missing runtime dependency on pip

[stanislavlevin]

According to docs https://flit.pypa.io/en/stable/cmdline.html#flit-install:

Flit calls pip to do the installation.

but pip is not specified as runtime dependency of flit.

[takluyver]

Hmmm, good point. I think I just assume pip is always present, which naturally is not really true. Do you want to make a PR?

[orsinium]

Project dependencies specify packages to be installed in the same environment as flit. However, flit currently uses pip installed in the target environment, which may be changed by passing --python flag. I'm going to change this behavior in #651 to make flit use pip from the flit's environment if available.