Pipfile shorthand markers: operator-less values become invalid marker text and extra is treated as a shorthand key #817

Follow-up to #815 (fixed in #816).

1. The shorthand key list includes extra. pipenv's PipenvMarkers has 11 keys and no extra, so extra = "== 'x'" should be ignored.
2. A shorthand value with no operator, for example python_version = "3.10", is emitted as the marker text python_version 3.10, which is not valid marker syntax. pipenv builds <key> <value>, and when the combined marker does not parse it drops the whole marker, so the requirement stays unconditional.

Source: https://github.com/pypa/pipenv/blob/c1739c9b/pipenv/utils/markers.py (PipenvMarkers.from_pipfile).

Fix: drop extra from the list and skip the marker when a shorthand value has no operator. One regression covers both.