chore(tests): simplify Python test dependencies

Remove version pinning from `pythonreq.txt` for test dependencies. This
allows flexibility in using the latest compatible versions of
`robotframework` and `robotframework-sshlibrary`. The change reduces
maintenance overhead while still ensuring compatibility for tests.

tests/pythonreq.txt

@@ -1,9 +1,2 @@
-robotframework==4.1.2
-robotframework-sshlibrary==3.8.0
-bcrypt==3.2.0
-cffi==1.15.0
-cryptography==36.0.1
-paramiko==2.9.2
-pycparser==2.21
-pynacl==1.5.0
-scp==0.14.2
+robotframework
+robotframework-sshlibrary