performance: fix a performance regression introduced in #402

[radoering]

Fixes a perfomance regression introduced in #402 by applying cached_property to certain properties

cached_property is available since Python 3.8. There is a package named backports.cached-property, which makes the decorator available in Python 3.7. However, I don't want to add a new vendored dependency since we will drop Python 3.7 support anyway in the near future. We could introduce a restricted dependency on backports.cached-property like in poetry or just accept the performance regression for Python 3.7. I have chosen the latter but do not have strong feelings about it.

In the following table you can see the cumtime of relevant calls (in seconds) from profiling different variants for a sample project

variant	excludes_single_version()	VersionUnion.allows()	VersionRangeConstraint.allowed_min
poetry master	2.2	0.3	-
poetry master with poetry-core master	15.3	12.4	7.7
poetry master with this PR	1.0	0.65	0.1

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