devel/py-pytest-asyncio: unbreak with python < 3.x by default

<phase: run-depends >

> py35-pytest-asyncio-0.5.0 depends on package: py35-pytest>=3.0.2 - not found

> py35-pytest-asyncio-0.5.0 depends on package: /packages/All/py35-pytest-3.0.3.txz - not found

> USE_PACKAGE_DEPENDS_ONLY set - not building missing dependency from source

• [run-depends] Error code 1

Reported by: pkg-fallout