2.9

Changes to the library:

• optimized the ArchiveEntry class (#94)
• added support for the zstd format (#90)
• fixed the top-level __all__ list (7b97d8b)
• updated the code to support Python 3.8 (#88)
• improved the add_file_from_memory method (#85 and #86)
• lowered the level of log messages from warning to info (#83)

Tests:

• dropped testing with Python 3.4, added 3.7 and 3.8 (#88)
• fixed unicode handling in entry tests (#84)