Log Message:
py-cyclonedx-python-lib: update to 2.0.0.

Feature

    Bump dependencies (da3f0ca)
    Completed work on #155 (#172) (a926b34)
    Support complete model for bom.metadata (#162) (2938a6c)
    Support for bom.externalReferences in JSON and XML #124 (1b733d7)
    Complete support for bom.components (#155) (32c0139)
    Support services in XML BOMs (9edf6c9)

Fix

    license_url not serialised in XML output #179 (#180) (f014d7c)
    Component.bom_ref is not Optional in our model implementation (in the schema \ 
it is) - we generate a UUID if bom_ref is not supplied explicitly (5c954d1)
    Temporary fix for __hash__ of Component with properties #153 (a51766d)
    Further fix for #150 (1f55f3e)
    Regression introduced by first fix for #150 (c09e396)
    Components with no version (optional since 1.4) produce invalid BOM output \ 
in XML #150 (70d25c8)
    expression not supported in Component Licsnes for version 1.0 (15b081b)

Breaking

    Adopt PEP-3102 (da3f0ca)
    Optional Lists are now non-optional Sets (da3f0ca)
    Remove concept of DEFAULT schema version - replaced with LATEST schema \ 
version (da3f0ca)
    Added BomRef data type (da3f0ca)