Add note about sandboxing to security implications section

Author: hoodmane

peps/pep-0776.rst

@@ -769,6 +769,9 @@ Security Implications
 
 Adding a new platform does not add any new security implications.
 
+Emscripten and WASI are also the only supported platforms that offer sandboxing.
+If users wish to execute untrusted Python code or untrusted Python extension
+modules, Emscripten provides a secure way for them to do that.
 
 How to Teach This
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