conda python breaks the python tests

For a user running through https://beam.apache.org/contribute/contribution-guide/#one-time-setup, the first time they run mvn verify the Python tests will fail if they have a conda python instance on their path.

To make the getting started experience easier for new devs we should call out that conda is not supported given its popularity.

cc [~altay]

Imported from Jira [BEAM-2087](https://issues.apache.org/jira/browse/BEAM-2087). Original Jira may contain additional context.
Reported by: holden.

Start with the one-time setup section of the contribution guide linked in the issue, then review how `mvn verify` invokes the Python tests. Update the getting-started guidance to clearly state that conda Python is unsupported, and verify that the documented setup communicates the limitation to new developers.

Written by the indexing model from the issue text.