Introduce UnknownSeries and UnknownIndex, type core.strings.pyi using them
#2888
| Job | Run time |
|---|---|
| 12s | |
| 3m 25s | |
| 3m 18s | |
| 4m 1s | |
| 5m 6s | |
| 5m 38s | |
| 4m 21s | |
| 6m 21s | |
| 8m 18s | |
| 2m 4s | |
| 2m 0s | |
| 3m 14s | |
| 3m 35s | |
| 51m 33s |