fix(deps): pin boto3 to 1.42.* on Python 3.9

boto3 1.43.0 requires Python >=3.10, which breaks installs on Python
3.9. Add a python_version gate so 3.9 stays on the 1.42.x series (the
last to support 3.9) while newer interpreters track 1.43.*.

requirements.txt

@@ -19,7 +19,8 @@ azure-mgmt-rdbms==10.1.1
 azure-mgmt-resource==25.0.0
 azure-mgmt-subscription==3.1.1
 bcrypt==5.0.*
-boto3==1.43.*
+boto3==1.42.*; python_version <= '3.9'
+boto3==1.43.*; python_version > '3.9'
 certifi==2026.4.22
 cryptography==47.0.*
 Flask-Babel==4.0.*