The Python version in Mojave is quite old (yes, 2.7, but behind in point releases (I have 2.7.10 whereas 2.7.16 is current (can’t conveniently check a freshly installed Mojave). And Python 2.anything reaches end of life (no more updates, security or otherwise) at the end of 2019. I don’t know what version is in Catalina (waiting for public beta).
The Python.org installers install into the writable area of Catalina (/usr/local/…) which is at least a good sign that they do it right (for what “right” is now).
I won’t have to keep doing my annual “I wonder what version of Python is included in macOS xxx” dance.
–John