ArcGIS Pro uses Python 3. This is a significant change from other ArcGIS products, including ArcGIS Desktop, ArcGIS Server, and ArcGIS Engine, that have used (and will continue to use) releases in the Python 2 line. The Python 3 line of releases are a different line that is incompatible with the Python 2 line. While fundamentally much of the language is the same, many key details have changed considerably, including how strings and dictionaries work and how the standard library is organized.
For tips in updating Python scripts to work in ArcGIS Pro, see Python 3 migration.