chore(deps): configure dependabot for pip deps

[corneliusroemer]

Relates to #1535

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[victorlin]

In the past I've tested dependabot.yml changes on a personal fork. I'll do the same here.

[corneliusroemer]

Excellent idea, I didn't know/think of that!

[victorlin]

Finally got around to testing this. Here is what it looks like on 2b497e6:

• Bump the prod-deps group with 4 updates victorlin/augur#6
• Bump the dev-deps group with 2 updates victorlin/augur#5

[victorlin]

Suggested change

	groups:
	dev-deps:
	dependency-type: "development"
	prod-deps:
	dependency-type: "production"

I think it'd be better to consider each update independently (i.e. remove the groups). That would change the Dependabot PRs from being grouped like this:

• Bump the prod-deps group with 4 updates victorlin/augur#6
• Bump the dev-deps group with 2 updates victorlin/augur#5

into this:

• Update pandas requirement from ==1.*,>=1.0.0 to >=1,<3 victorlin/augur#11
• Update jsonschema requirement from ==3.*,>=3.0.0 to >=3,<5 victorlin/augur#10
• Update numpy requirement from ==1.* to >=1,<3 victorlin/augur#9
• Update networkx requirement from ==2.*,>=2.5 to >=2,<4 victorlin/augur#8
• Update pandas-stubs requirement from ==1.*,>=1.0.0 to >=1,<3 victorlin/augur#7

[victorlin]

@corneliusroemer do you mind if I apply the suggestion above and merge?