Remove 'keep_tz' from '_calc_pathlength' function in metrics.py

[abhisheksparikh]

I got the following warning when using the variability_index function:

FutureWarning: The 'keep_tz' keyword in DatetimeIndex.to_series is deprecated and will be removed in a future version. You can stop passing 'keep_tz' to silence this warning.

I would like to remove this and see what the effects are. Thoughts?

[cwhanse]

Thanks for mentioning this. I would say to keep it until it's removed; someone may be using Series without localized datetimes. Please leave the issue open to remind us to remove it in the future.