`TIMESTAMPTZ` → Python conversion (`fetchone`/`.df()`/`.arrow()`) returns wrong DST offset for dates from 2038 onward (pytz Year 2038 bug)

### What happens?

When a `TIMESTAMPTZ` value is materialized into Python — via `fetchone()`/`fetchall()`, `.df()`, or `.arrow()` — for any date **from 2038 onward** during a DST period, the returned offset is the zone's **standard** offset instead of its **DST** offset. The underlying stored value (the UTC instant / epoch) is correct; only the Python-side conversion is wrong.

This makes `.df()`/`.arrow()` results silently wrong by exactly one hour for any DST-period timestamp dated 2038+ — the local wall-clock time attached to the returned value is shifted, even though `SELECT ... ::VARCHAR` and `... AT TIME ZONE 'UTC'` both show the correct value for the same row.

I traced this to `duckdb::PytzCacheItem` in the compiled Python extension (symbol visible via `strings` on `_duckdb.cpython-*.so`) — DuckDB's Python client builds the returned `tzinfo` via `pytz`, and `pytz` has a known, still-unresolved Year 2038 defect in its DST-transition lookup for zone-name-based localization. I confirmed the identical wrong offset by calling `pytz` directly, with no DuckDB involved at all (see repro below) — so this isn't a DuckDB-specific timezone calculation bug, it's DuckDB inheriting a known `pytz` defect through its Python conversion layer.

This is the same class of defect as the open Apache Arrow issue [apache/arrow#36110](https://github.com/apache/arrow/issues/36110) ("Wrong result when converting time zones after 2038"), which affects PyArrow's `assume_timezone()` for the same reason (offset resolution breaking at the 32-bit Unix-time boundary, 2038-01-19T03:14:07Z). We hit this in production after having already migrated *away* from DuckDB's `make_timestamptz()` to PyArrow specifically to avoid this bug class, only to discover PyArrow has an identical defect — and now find DuckDB's own Python client reintroduces it through `pytz`.

### To Reproduce

```python
import duckdb

con = duckdb.connect()
con.execute("INSTALL icu")
con.execute("LOAD icu")
con.execute("SET TimeZone='America/Chicago'")

for year in (2037, 2038, 2039):
sql_repr = con.execute(
f"SELECT make_timestamptz({year}, 8, 1, 3, 0, 0)::VARCHAR"
).fetchone()[0]
py_obj = con.execute(
f"SELECT make_timestamptz({year}, 8, 1, 3, 0, 0)"
).fetchone()[0]
print(year, "| SQL:", sql_repr, "| Python:", repr(py_obj))
```

Output (DuckDB 1.5.3, `pytz` 2026.2):

```
2037 | SQL: 2037-08-01 03:00:00-05 | Python: datetime.datetime(2037, 8, 1, 3, 0, tzinfo=)
2038 | SQL: 2038-08-01 03:00:00-05 | Python: datetime.datetime(2038, 8, 1, 2, 0, tzinfo=)
2039 | SQL: 2039-08-01 03:00:00-05 | Python: datetime.datetime(2039, 8, 1, 2, 0, tzinfo=)
```

Note the SQL-level `VARCHAR` cast correctly shows `-05` (CDT) for all three years, but the Python `datetime` object returned for the same row silently switches to `CST`/`-06:00` starting in 2038, and the local hour is wrong (`2` instead of `3`) as a result.

Same bug reproduces via `.df()` and `.arrow()`:

```python
df = con.execute(
"SELECT year, make_timestamptz(year, 8, 1, 3, 0, 0) AS ts "
"FROM (VALUES (2037),(2038),(2039)) t(year)"
).df()
print(df)
# year ts
# 0 2037 2037-08-01 03:00:00-05:00
# 1 2038 2038-08-01 02:00:00-06:00 <- wrong, should be 03:00:00-05:00
# 2 2039 2039-08-01 02:00:00-06:00 <- wrong, should be 03:00:00-05:00
```

`con.execute(...).arrow()` on the same query produces identical (wrong) values.

For comparison, calling `pytz` directly — no DuckDB involved — reproduces the exact same wrong offset, confirming the root cause:

```python
import pytz
from datetime import datetime, timezone

tz = pytz.timezone("America/Chicago")
for year in (2037, 2038, 2039):
utc_dt = datetime(year, 8, 1, 8, 0, 0, tzinfo=timezone.utc)
print(year, utc_dt.astimezone(tz))
# 2037 2037-08-01 03:00:00-05:00
# 2038 2038-08-01 02:00:00-06:00 <- wrong
# 2039 2039-08-01 02:00:00-06:00 <- wrong
```

And with stdlib `zoneinfo` instead of `pytz`, the correct offset is returned for every year:

```python
import zoneinfo
tz = zoneinfo.ZoneInfo("America/Chicago")
for year in (2037, 2038, 2039):
utc_dt = datetime(year, 8, 1, 8, 0, 0, tzinfo=timezone.utc)
print(year, utc_dt.astimezone(tz))
# 2037 2037-08-01 03:00:00-05:00
# 2038 2038-08-01 03:00:00-05:00 <- correct
# 2039 2039-08-01 03:00:00-05:00 <- correct
```

### OS:

Linux

### DuckDB Version:

1.5.3 (Python client `duckdb==1.5.3`)

### DuckDB Client:

Python

### Hardware:

_No response_

### Full Name:

Marc Macleod

### Affiliation:

Ascend Analytics

### Did you include all relevant configuration (e.g., CPU architecture, Linux distribution) to reproduce the issue?

- [x] Yes, I have

### Did you include all code required to reproduce the issue?

- [x] Yes, I have

### Did you include all relevant data sets for reproducing the issue?

Yes