Make Timezone instances built via from_file() picklable

[afonsojanu]

Closes #899.

get_local_timezone()'s last-resort fallback reads the raw zoneinfo file at /etc/localtime (or the file the TZ environment variable points at) directly, since the system's timezone name couldn't be derived any other way. That goes through Timezone.from_file(), and the underlying zoneinfo.ZoneInfo.from_file() refuses to pickle any instance built that way, key or no key, since it has no record of which file it came from to reconstruct on unpickling.

import pickle, pendulum
dt = pendulum.now()
pickle.dumps(dt)
# _pickle.PicklingError: Cannot pickle a ZoneInfo file from a file stream.

This reproduces whenever the local timezone name can't be identified (as reported in the issue), so the local tzinfo ends up being one of these keyless, file-based instances.

Timezone.from_file() now keeps the raw TZif bytes it read around on the instance, and __reduce__ uses them to rebuild an equivalent instance on unpickling instead of delegating to the version inherited from zoneinfo.ZoneInfo, which unconditionally rejects this. Instances built via the regular Timezone(key) constructor still pickle exactly as before, by key.

Added two tests covering both cases (keyless from_file(), and from_file() with a key), using the raw TZif bytes from the tzdata package (already a dependency) so the test works the same on every platform, including Windows, which has no /usr/share/zoneinfo at all.