www/py-soupsieve: Add missing dependency
soupsieve depends on backports.functools_lru_cache  on Python versions <
3, add the dependency accordingly.
While I'm here:
- Add TEST_DEPENDS and test target to improve future QA
Note: soupsieve 'requires'  beautifulsoup4, but beautifulsoup4 depends
on soupsieve via install_requires, causing a technical circular dependency, which is why soupsieve doesn't seem to explicitly install_requires on it ... anymore . This seems bizzare.
 requirements/project.txt: backports.functools_lru_cache ; python_version < '3'
QA: 624 passed, 2 skipped, 6 warnings in 35.70 seconds
Reported by: jbeich
Approved by: portmgr (blanket: missing dependencies, ports compliance)
MFH: 2019Q3 (blanket(s): run-time bugfix)