Replace nose2 with pytest.

[purple4reina]

What does this PR do?

Replaces nose2 with pytest for testing

Motivation

Pytest is the most widely used python test runner

Testing Guidelines

Just run the tests

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

https://datadoghq.atlassian.net/browse/SVLS-4152