CMake doesn't check for ruamel.yaml Python Module

[bpadalino]

CMake doesn't check if the ruamel.yaml Python module is installed. UHD checks for it in the following way:

set(RUAMEL.YAML_MIN_VERSION "0.15")
PYTHON_CHECK_MODULE(
    "ruamel.yaml ${RUAMEL.YAML_MIN_VERSION} or greater"
    "ruamel.yaml" "LooseVersion(ruamel.yaml.__version__) >= LooseVersion('${RUAMEL.YAML_MIN_VERSION}')"
    HAVE_PYTHON_MODULE_YAML
)

Where PYTHON_CHECK_MODULE comes from UHDPython.cmake which isn't installed with UHD, but comes from UHD.

In some test files (python_api_test.py and uhd_test_base.py), UHD does the following:

try:
    from ruamel import yaml
except:
    import yaml

Is the ruamel.yaml Python module really required, or is it just nice to have? Can the try/except import work for this dependency? If not, should UHDPython.cmake be installed by UHD, or should the macro just come over to this repository?