2.1.3

Fix an issue where the union of specific inverse or partially inverse markers was not simplified (#858).
Fix an issue where optional dependencies defined in the project section were treated as non-optional when a source was defined for them in the tool.poetry section (#857).
Fix an issue where markers with === were not parsed correctly (#860).
Fix an issue where local versions with upper case letters caused an error (#859).
Fix an issue where extra markers with a value starting with "in" were not validated correctly (#862).
Fix an issue where inheriting from WheelBuilder was unnecessarily difficult (#855).

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