Add change note for 1.21.

Author: taus-semmle

change-notes/1.21/analysis-python.md

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+# Improvements to Python analysis
+
+
+## General improvements
+
+> Changes that affect alerts in many files or from many queries
+> For example, changes to file classification
+
+## New queries
+  | **Query** | **Tags** | **Purpose** |
+  |-----------|----------|-------------|
+  | Accepting unknown SSH host keys when using Paramiko. (`py/paramiko-missing-host-key-validation`) | security, external/cwe/cwe-295 | Finds instances where Paramiko is configured to accept unknown host keys. Results are shown on LGTM by default. |
+
+
+## Changes to existing queries
+
+  | **Query** | **Expected impact** | **Change** |
+  |-----------|---------------------|------------|
+
+## Changes to code extraction
+
+* *Series of bullet points*
+
+## Changes to QL libraries
+
+* *Series of bullet points*