python,python3: define PyBuild/Compile & Py3Build/Compile

python,python3: define PyBuild/Compile & Py3Build/Compile

Similar to LEDE/OpenWrt's Build/Compile/Default rule,
and other similarities like this.

This should allow Python packages to define
PyBuild/Compile rules to do specific stuff per
package.

The advantage of using these (over just overriding
Build/Compile) is the VARIANT mechanism that is
in place to support packaging both for Python & Python3.

So, PyBuild/Compile will get picked up for the Python
variant build, and Py3Build/Compile will get picked
up for the Python3 variant build.

Signed-off-by: Alexandru Ardelean <[email protected]>