Fix pendulum.tz.timezones() to use system tzdata (#801)

* Fix `pendulum.tz.timezones()` to use system tzdata

Fix the `pendulum.tz.available_timezones()` to use
`available_timezones()` function instead of iterating over the files
in `tzdata` package.  This is more in line with PEP 615, as the system
timezone functions will operate on system-provided tzdata when
available, and use the `tzdata` package only if it's not available.
Therefore, the previous code would yield a potentially different list
of timezones than the system actually provides.

Furthermore, Gentoo provides a dummy `tzdata` package that does not
provide any data, since Python always uses system tzdata.  This change
is necessary to make pendulum work again on Gentoo.

Fixes #769

* Fix type hints

* Remove unused `_timezones` variable

* Remove unused imports

* Add caching to `timezones()`

Author: mgorny

src/pendulum/tz/__init__.py

@@ -1,8 +1,7 @@
 from __future__ import annotations
 
-from importlib import resources
-from typing import TYPE_CHECKING
-from typing import cast
+from functools import cache
+from zoneinfo import available_timezones
 
 from pendulum.tz.local_timezone import get_local_timezone
 from pendulum.tz.local_timezone import set_local_timezone
@@ -12,27 +11,16 @@
 from pendulum.tz.timezone import Timezone
 
 
-if TYPE_CHECKING:
-    from pathlib import Path
-
-
 PRE_TRANSITION = "pre"
 POST_TRANSITION = "post"
 TRANSITION_ERROR = "error"
 
-_timezones = None
-
 _tz_cache: dict[int, FixedTimezone] = {}
 
 
-def timezones() -> tuple[str, ...]:
-    global _timezones
-
-    if _timezones is None:
-        with cast("Path", resources.files("tzdata").joinpath("zones")).open() as f:
-            _timezones = tuple(tz.strip() for tz in f.readlines())
-
-    return _timezones
+@cache
+def timezones() -> set[str]:
+    return available_timezones()
 
 
 def fixed_timezone(offset: int) -> FixedTimezone: