build: depends on pysnmp-lextudio < 6

The cmdgen API has been wiped out. I suppose everything needs to be
rewritten to use hlapi instead.

setup.py

@@ -52,7 +52,7 @@ def run_tests(self, *args, **kwds):
         cffi_modules=(not rtd and ["snimpy/smi_build.py:ffi"] or []),
         install_requires=[
             "cffi >= 1.0.0",
-            "pysnmp-lextudio >= 4",
+            "pysnmp-lextudio >= 4, < 6",
             'pyasyncore; python_version >= "3.12"',
             "setuptools",
         ],