fix(markers): when left hand value is non-version string, fallback to python behavior

[frostming]

Signed-off-by: Frost Ming me@frostming.com

[notatallshaw]

This correctly follows the spec but was previously rejected: #816

[frostming]

Good to know, that is quite unfortunate.

[brettcannon]

This probably requires #774 to get resolved overall before this can be merged.

[henryiii]

The behavior of this has changed, it used to raise a InvalidVersion error, but now it returns False always, due to changes in SpecifierSet with invalid versions. As the spec still hasn't changed, would it make sense to go ahead and put this in to match the current spec vs. changing from one non-compliant behavior to another?

(this idea, not this change exactly, since it now doesn't raise an InvalidVersion)

[henryiii]

Closing since #939 was accepted and will be in the RC (coming soon). Please test and reopen if consideration on this still is needed. Combined with other changes, hopefully this all works.