ENH: Move to requiring python 3

Resolves #1081

See for more details:
https://devguide.python.org/devcycle/#end-of-life-branches
https://pythonclock.org/

src/jsontestrunner/CMakeLists.txt

@@ -1,4 +1,13 @@
-find_package(PythonInterp 2.6)
+if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.12.0)
+  # The new Python3 module is much more robust than the previous PythonInterp
+  find_package (Python3 COMPONENTS Interpreter)
+  # Set variables for backwards compatibility with cmake < 3.12.0
+  set(PYTHONINTERP_FOUND ${Python3_Interpreter_FOUND})
+  set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
+else()
+  set(Python_ADDITIONAL_VERSIONS 3.8)
+  find_package(PythonInterp 3)
+endif()
 
 add_executable(jsontestrunner_exe
                main.cpp