Fix Marker non-version-to-version eval failing

As described in my previous commit to add test cases covering this
behaviour, PEP 508 - Environment Markers section specifies that markers
are evaluated using version operator rules when both sides are valid
versions, and otherwise regular python operator rules apply.

However, the implementation was failing to handle the case where the RHS
was a valid version specifier, but the LHS was not a valid version. In
this case, PEP 508 requires the comparison falls back to python rules,
but the implementation was throwing InvalidVersion.

The implementation now matches the behaviour in the spec, as we now
check that both the LHS and the RHS are versions before attempting
version comparison (not just the RHS as before).

Author: h4l

src/packaging/markers.py

@@ -15,6 +15,7 @@
 from ._tokenizer import ParserSyntaxError
 from .specifiers import InvalidSpecifier, Specifier
 from .utils import canonicalize_name
+from .version import InvalidVersion, Version
 
 __all__ = [
     "InvalidMarker",
@@ -175,12 +176,20 @@ def _format_marker(
 
 
 def _eval_op(lhs: str, op: Op, rhs: str) -> bool:
+    # PEP 508 - Environment Markers
+    # https://peps.python.org/pep-0508/#environment-markers
+    # > The <version_cmp> operators use the PEP 440 version comparison rules
+    # > when those are defined (that is when both sides have a valid version
+    # > specifier). If there is no defined PEP 440 behaviour and the operator
+    # > exists in Python, then the operator falls back to the Python behaviour.
+    # > Otherwise an error should be raised.
     try:
         spec = Specifier("".join([op.serialize(), rhs]))
-    except InvalidSpecifier:
+        lhs_ver = Version(lhs)
+    except (InvalidSpecifier, InvalidVersion):
         pass
     else:
-        return spec.contains(lhs, prereleases=True)
+        return spec.contains(lhs_ver, prereleases=True)
 
     oper: Operator | None = _operators.get(op.serialize())
     if oper is None: