Admin: Improve UX when Flask dependencies are missing (#5499)

* Revert "Dependencies - always include Flask"

This reverts commit c1a4d0521203370738c02f6ced23bffb6aa724d6.

* UX - Warn user when missing Flask dependencies

moto/moto_server/werkzeug_app.py

@@ -3,8 +3,16 @@ import os
 import os.path
 from threading import Lock
 
-from flask import Flask
-from flask_cors import CORS
+try:
+    from flask import Flask
+    from flask_cors import CORS
+except ImportError:
+    import warnings
+
+    warnings.warn(
+        "When using MotoServer, ensure that you install moto[server] to have all dependencies!\n"
+    )
+    raise
 
 import moto.backends as backends
 import moto.backend_index as backend_index

setup.py

@@ -39,8 +39,6 @@ install_requires = [
     "MarkupSafe!=2.0.0a1",  # This is a Jinja2 dependency, 2.0.0a1 currently seems broken
     "Jinja2>=2.10.1",
     "importlib_metadata ; python_version < '3.8'",
-    "flask<2.2.0",  # Required for the ThreadedMotoServer
-    "flask-cors"
 ]
 
 _dep_PyYAML = "PyYAML>=5.1"
@@ -75,7 +73,7 @@ all_extra_deps = [
     _dep_openapi,
     _setuptools,
 ]
-all_server_deps = all_extra_deps
+all_server_deps = all_extra_deps + ["flask<2.2.0", "flask-cors"]
 
 extras_per_service = {}
 for service_name in [