v3.17.9

Commits:

• 1d16205 feat(enum): return enum from object keys (#1216)
• 8b9daba Don't use invalid_type_error for undefined fields. (#1175)
• 0348e9b Fixed typing mistake in README.md (#1160)
• 15aa4ab Convert "an nullable" to "a nullable" (#1162)
• 55d2744 3.17.9