Lock dependencies using pip-compile

[victorlin]

Currently, dependencies are defined rather loosely:

ncov-ingest/requirements.txt

Lines 1 to 8 in e6c904f

	nextstrain-cli>=2.0.0
	csv-diff>=0.6
	pandas>=1.0.1
	biopython
	regex
	envdir
	xopen
	typer

From #416 (comment):

Ideally, it would still be nice to lock these deps so they're not changing and liable to break on new upstream releases updates. I'd use pip-compile from pip-tools for that.