Update docs for python 3 and remove python 2

[tst-cjeffries]

Updates documentation to reflect python 3 migration. Removes python 2 and references to it.

To test:

• Review docs that were changed.
• Build and install OpenGEE according to build instructions (https://github.com/tst-cjeffries/earthenterprise/blob/remove_python2/earth_enterprise/BUILD_RHEL_CentOS.md)