build: too recent pyasn1 broke pysnmp

Sigh... It should be a push to update the code to make it work with more
recent versions of PySNMP.

setup.py

@@ -53,6 +53,7 @@ def run_tests(self, *args, **kwds):
         install_requires=[
             "cffi >= 1.0.0",
             "pysnmp-lextudio >= 4, < 6",
+            "pyasn1 <= 0.6.0",
             'pyasyncore; python_version >= "3.12"',
             "setuptools",
         ],