feat!: Change str.to_datetime to default to microsecond precision for format specifiers "%f" and "%.f"

[MarcoGorelli]

Closes #13592

Changes

• In .str.to_datetime, when specifying %.f as the format, the default was to set the resulting datatype to nanosecond precision. This will now be changed to microsecond precision.

Example

Before

>>> s = pl.Series(["2022-08-31 00:00:00.123456789"])
>>> s.str.to_datetime(format="%Y-%m-%d %H:%M:%S%.f")
shape: (1,)
Series: '' [datetime[ns]]
[
        2022-08-31 00:00:00.123456789
]

After

>>> s.str.to_datetime(format="%Y-%m-%d %H:%M:%S%.f")
shape: (1,)
Series: '' [datetime[us]]
[
        2022-08-31 00:00:00.123456
]

[stinodego]

Seems like a good idea 👍