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

src/pendulum/tz/__init__.py

@@ -9,7 +9,7 @@
 from pendulum.tz.timezone import UTC
 from pendulum.tz.timezone import FixedTimezone
 from pendulum.tz.timezone import Timezone
-from pendulum.utils._compat import resources
+from pendulum.utils._zoneinfo import available_timezones
 
 
 PRE_TRANSITION = "pre"
@@ -22,13 +22,7 @@
 
 
 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
+    return available_timezones()
 
 
 def fixed_timezone(offset: int) -> FixedTimezone: