[Python] Use a python_version marker instead of a condition

[asfimport]

When installing pyarrow 0.9.0 pipenv complains that futures has no matching versions.

While that may be a bug in pipenv it does not matter. The standard way to specify a conditional dependency is using a marker.

We should use the python_version marker to tell pip if it should install futures or not.

Reporter: Omer Katz / @thedrow
Assignee: Omer Katz / @thedrow

PRs and other links:

• GitHub Pull Request #1879

_{Note: This issue was originally created as ARROW-2464. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 1879
#1879