DEPR: Enforce ewm(times=str) deprecation
Introduced in https://github.com/pandas-dev/pandas/pull/43265

Test that it raises? Whatsnew? > Test that it raises? Whatsnew?

Added test here https://github.com/pandas-dev/pandas/pull/48842/files#diff-3f0452bb29c5fda1dfc3fb1ec8346ac3a1fffe504c857d232c1df7a043b45ad8R239