Missing f-string in logger warning

In `chaco/axis.py`, `PlotAxis._compute_tick_positions()` the `logger.warning` message written `if datalow > datahigh` is using regular strings instead of f-strings, so the warning message is not formatted correctly.

It should be:
```python
if datalow > datahigh:
logger.warning(
f"{self.mapper} has an invalid data range with "
f"low={datalow} > high={datahigh}; unable to compute axis "
"ticks."
)
```