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* Issue 240: Based on real-world performance measures against 5.4, zip
manifests are now cached in all circumstances. The
``PKG_RESOURCES_CACHE_ZIP_MANIFESTS`` environment variable is no longer
relevant. The observed "memory increase" referenced in the 5.4 release
notes and detailed in Issue 154 was likely not an increase over the status
quo, but rather only an increase over not storing the zip info at all.