[gdb/dap] Disable DAP for python <= 3.5

DAP requires python module typing, which is supported starting python 3.5.

Make this formal by:
- disabling the dap interpreter for python version < 3.5
- returning 0 in allow_dap_tests for python version < 3.5

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

PR dap/30708
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30708

gdb/python/py-dap.c

@@ -91,8 +91,11 @@ void _initialize_py_interp ();
 void
 _initialize_py_interp ()
 {
+  /* The dap code uses module typing, available starting python 3.5.  */
+#if PY_VERSION_HEX >= 0x03050000
   interp_factory_register ("dap", [] (const char *name) -> interp *
     {
       return new dap_interp (name);
     });
+#endif
 }

gdb/testsuite/lib/gdb.exp

@@ -1533,6 +1533,26 @@ proc gdb_test { args } {
     return [gdb_test_multiple $command $message {*}$opts $user_code]
 }
 
+# Return 1 if python version used is at least MAJOR.MINOR
+proc python_version_at_least { major minor } {
+    set python_script {print (sys.version_info\[0\], sys.version_info\[1\])}
+
+    set res [remote_exec host $::GDB \
+		 "$::INTERNAL_GDBFLAGS -batch -ex \"python $python_script\""]
+    if { [lindex $res 0] != 0 } {
+	error "Couldn't get python version"
+    }
+
+    set python_version [lindex $res 1]
+    set python_version [string trim $python_version]
+
+    regexp {^([0-9]+) ([0-9]+)$} $python_version \
+	dummy python_version_major python_version_minor
+
+    return [version_compare [list $major $minor] \
+		<= [list $python_version_major $python_version_minor]]
+}
+
 # Return 1 if tcl version used is at least MAJOR.MINOR
 proc tcl_version_at_least { major minor } {
     global tcl_version
@@ -2586,6 +2606,11 @@ gdb_caching_proc allow_dap_tests {} {
 	return 0
     }
 
+    # The dap code uses module typing, available starting python 3.5.
+    if { ![python_version_at_least 3 5] } {
+	return 0
+    }
+
     # ton.tcl uses "string is entier", supported starting tcl 8.6.
     if { ![tcl_version_at_least 8 6] } {
 	return 0