'%+' isn't auto-detected to be tz-aware

[MarcoGorelli]

Polars version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of Polars.

Issue description

'%+' requires a timezone to be present, but polars doesn't auto-detect it to be tz-aware

Reproducible example

In [1]: pl.Series(["2020-01-01T00:00:00Z"]).str.strptime(pl.Datetime, '%+')
Out[1]:
shape: (1,)
Series: '' [datetime[μs]]
[
        2020-01-01 00:00:00
]

Expected behavior

shape: (1,)
Series: '' [datetime[μs, +00:00]]
[
        2020-01-01 00:00:00 +00:00
]

Installed versions

---Version info--- Polars: 0.15.16 Index type: UInt32 Platform: Linux-5.10.102.1-microsoft-standard-WSL2-x86_64-with-glibc2.35 Python: 3.11.1 (main, Dec 7 2022, 01:11:34) [GCC 11.3.0] ---Optional dependencies--- pyarrow: 10.0.1 pandas: 1.5.3 numpy: 1.24.1 fsspec: connectorx: xlsx2csv: deltalake: matplotlib: