fix: Use typing_extensions for Self in Python < 3.11

Adds typing_extensions as a dependency and uses a conditional
import for the Self type in src/crewai/memory/memory.py to
ensure compatibility with Python 3.10, resolving the
ImportError seen in CI.

Refs #2575

Co-Authored-By: Joe Moura <joao@crewai.com>

pyproject.toml

@@ -38,6 +38,7 @@ dependencies = [
     "blinker>=1.9.0",
     "json5>=0.10.0",
 ]
+    "typing-extensions>=4.7.0", # For Self type backport
 
 [project.urls]
 Homepage = "https://crewai.com"