Add filelock as a dependency

[mariusvniekerk]

This seems to be needed for some classes of pip based resolution

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[maresb]

As of four months ago, filelock >=3.8.0 should already be implicit in cachecontrol[filecache]. But explicit is better than implicit.

I'm concerned about removing the pip-support option and breaking pipelines, but a quick experiment shows that pip only emits a warning, so I think we're good.

[maresb]

Closes #299