ports/unix: Enable enum package in coverage and standard variants.

Adds enum package from micropython-lib to unix port variants:
- coverage variant: For testing metaclass and enum features
- standard variant: For general enum availability

The enum package provides Enum, IntEnum, Flag, IntFlag, StrEnum,
and auto() compatible with CPython's enum module.

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl

ports/unix/variants/coverage/manifest.py

@@ -3,3 +3,4 @@
 freeze_as_mpy("frzmpy")
 freeze_mpy("$(MPY_DIR)/tests/frozen")
 require("ssl")
+require("enum")

ports/unix/variants/standard/manifest.py

@@ -1,3 +1,6 @@
 include("$(PORT_DIR)/variants/manifest.py")
 
 include("$(MPY_DIR)/extmod/asyncio")
+
+# Enum module from micropython-lib
+require("enum")