Add a test for `GET /connection/:connection_id`

[sajith]

Resolves #270. Changes:

• Add a test case with topology and connection setup. In this test we can expect a 200 response when doing a GET /connection/:connection_id.
• Update testing instructions in README.
• Bump up black version in pyproject.toml to be the same as CI.

Although #270 is assigned to @italovalcy, I made an attempt at it. ;-)

[coveralls]

Pull Request Test Coverage Report for Build 9112570287

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 51.721%

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Totals
Change from base Build 9112522793:	0.1%
Covered Lines:	725
Relevant Lines:	1433

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💛 - Coveralls

[sajith]

The interesting thing about this particular untidiest is why it didn't fail before? I was concerned if I broke sth I didn't notice.

I think GET /connection/:connection_id was a stub that returned success. And then we added a DB lookup.