[gdb/dap] Fix dap for python < 3.8

With any gdb.dap test and python 3.6 I run into:
...
Error occurred in Python: 'code' object has no attribute 'co_posonlyargcount'
ERROR: eof reading json header
...

The attribute is not supported before python 3.8, which introduced the
"Positional−only Parameters" concept.

Fix this by using try/except AttributeError.

Tested on x86_64-linux:
- openSUSE Leap 15.4 with python 3.6, and
- openSUSE Tumbleweed with python 3.11.5.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/python/lib/gdb/dap/server.py

@@ -168,7 +168,11 @@ def request(name):
         global _commands
         code = func.__code__
         # We don't permit requests to have positional arguments.
-        assert code.co_posonlyargcount == 0
+        try:
+            assert code.co_posonlyargcount == 0
+        except AttributeError:
+            # Attribute co_posonlyargcount is supported starting python 3.8.
+            pass
         assert code.co_argcount == 0
         # A request must have a **args parameter.
         assert code.co_flags & inspect.CO_VARKEYWORDS