python.mk: move ABI flag removal modifier out of ${_VC}

Ugly hack needed by older bmake versions, ie not -CURRENT:

: bmake: "/usr/ports/Mk/Uses/python.mk" line 471: Unknown modifier "/"
: bmake: "/usr/ports/Mk/Uses/python.mk" line 471: Malformed conditional (!empty(_PYTHON_VERSION_MAXIMUM) && (${_PYTHON_VERSION:${_VC}} > ${_PYTHON_VERSION_MAXIMUM:${_VC}}))

Fixes: ccaff6c14f2078ddc0edcbf26d7ee7e48b80c2a8
Reported by: cmt
PR: 274671
Event: Kitchener-Waterloo Hackathon 202506