pickle5 backport for older Python

pickle5 is a backport of the pickle protocol 5 (PEP 574) and other related changes, primarily for Python versions prior to 3.8. It provides the performance improvements and features of protocol 5 for Python 3.5, 3.6, and 3.7. The library is currently at version 0.0.12 and is no longer actively maintained, as its purpose has been superseded by native support in Python 3.8+.