python: Allow newer prettytable with newer python

This helps to allow installation of Mbed OS's Python requirements, when
CMake is used, into the same Python environment as mbed-os-tools. The
versions specified here are aligned with mbed-os-tools;
tools/cmake/requirements.txt depends on the same version of prettytable
as mbed-os-tools's requirements.txt.

Author: Patater

tools/cmake/requirements.txt

@@ -1,4 +1,5 @@
-prettytable==0.7.2
+PrettyTable<=1.0.1; python_version < '3.6'
+prettytable>=2.0,<3.0; python_version >= '3.6'
 future>=0.18.0,<1.0
 jinja2>=2.11.3
 intelhex>=2.3.0,<3.0.0