Unpin coverage

[sajith]

#3267 pinned some dependencies including coverage, because integration tests were failing on CI. It isn't clear if coverage still needs to remain pinned at ~= 4.5.

Let us unpin coverage and see what happens in CI!

For more context, the follow-up ticket to this would be #3385, which will attempt to use ​coveralls.io for code coverage checks. We are not entirely satisfied with ​codecov.io, which we currently use. For example, it is not always clear why codecov checks turn red/disapprove of certain PRs, even when test coverage remains unchanged.

Now, in order to upload coverage reports to codecov.io, we will need to use ​coveralls-python, which requires that coverage reports should be in coverage 5.0 format. Which is the real motivation for unpinning coverage.

If coveralls.io works better than codecov.io for us, we can switch to the former. Even if it doesn't, this change does not affect codecov: codecov can continue working with the reports we submit in XML format.

(See coverage ​changelog: "Coverage’s data storage has changed. In version 4.x, .coverage files were basically JSON. Now, they are SQLite databases.")