MNT: Replace lazyproperty with cached_property in Python 3.8

As @Mariatta pointed out in https://github.com/astropy/astropy/pull/8881#issuecomment-513956394 , we might be able to replace `lazyproperty` with [cached_property](https://docs.python.org/3.8/library/functools.html?highlight=cached_property#functools.cached_property) that is new in Python 3.8. We also need to figure out the best way to support this for Python<=3.7 or wait until our Python minversion is 3.8.

Review PR #8881 and the Python 3.8 functools.cached_property documentation first. Determine how support for Python <=3.7 should work, or whether the project should wait until Python 3.8 is the minimum version. Done means an agreed replacement strategy that covers the supported Python versions.

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