tests/qapi-schema: Use Python OSError instead of outmoded IOError

https://docs.python.org/3.6/library/exceptions.html has

    Changed in version 3.3: EnvironmentError, IOError, WindowsError,
    socket.error, select.error and mmap.error have been merged into
    OSError, and the constructor may return a subclass.

and

    The following exceptions are kept for compatibility with previous
    versions; starting from Python 3.3, they are aliases of OSError.

    exception EnvironmentError

    exception IOError

    exception WindowsError

        Only available on Windows.

Switch to the preferred name.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210922125619.670673-2-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[Details added to commit message]

tests/qapi-schema/test-qapi.py

@@ -154,7 +154,7 @@ def test_and_diff(test_name, dir_name, update):
         errfp = open(os.path.join(dir_name, test_name + '.err'), mode)
         expected_out = outfp.readlines()
         expected_err = errfp.readlines()
-    except IOError as err:
+    except OSError as err:
         print("%s: can't open '%s': %s"
               % (sys.argv[0], err.filename, err.strerror),
               file=sys.stderr)
@@ -180,7 +180,7 @@ def test_and_diff(test_name, dir_name, update):
         errfp.truncate(0)
         errfp.seek(0)
         errfp.writelines(actual_err)
-    except IOError as err:
+    except OSError as err:
         print("%s: can't write '%s': %s"
               % (sys.argv[0], err.filename, err.strerror),
               file=sys.stderr)