Default Python version is bad

[varunagrawal]

Description

The default Python version in CMake is Default. When turning on the GTSAM_BUILD_PYTHON flag, the Default values causes an error with Pybind11.

Found PythonInterp: /home/varun/.pyenv/shims/python (found version "3.8.5")
 Found PythonLibs: /usr/lib/libpython3.8.so (found version "3.8.5")
 CMake Error at wrap/pybind11/tools/pybind11Tools.cmake:16 (find_package):
   find_package called with invalid argument "Default"
 Call Stack (most recent call first):
   wrap/pybind11/CMakeLists.txt:33 (include)

 pybind11 v2.5.dev1

Setting the Python version to a specific version, such as 3.8, fixes the issue, so the proposed solution would be to set the default to empty ("") and use the CMake variables from Python to set the default version value. @dellaert @ProfFan

Steps to reproduce

1. Use fresh build directory.
2. Set the GTSAM_BUILD_PYTHON flag to ON.
3. Run cmake ..

Expected behavior

CMake should not throw an error and should configure the build without issues.

Environment

• OS: Arch Linux
• Clang compiler