Pendulum shouldn't rely on Linux distro-specific timezone configuration

[satwell]

• [x] I have searched the issues of this repo and believe that this is not a duplicate.
• [x] I have searched the documentation and believe that my question is not covered.

Issue

I see in pendulum.tz.local_timezone that Pendulum tries to get the local timezone from some distro-specific files such as /etc/timezone and /etc/sysconfig/clock. It only falls back to /etc/localtime if these files don't exist. But libc only uses /etc/localtime to get the system default local timezone. (glibc source is hard to follow, but this is pretty clear in do_tzset() in musl libc.)

This can lead to weird behavior where most apps (e.g., the date command) think the system uses one timezone, but Pendulum thinks it uses a different one. For example, one way that people sometimes override the timezone in Docker containers is to bind mount /etc/localtime from the host system into the container. Which can result in a mismatch between /etc/localtime and /etc/timezone but works fine for almost everything because libc only uses /etc/localtime.

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=822733 where Debian is working on getting rid of /etc/timezone.

It seems like it would be best to remove handling of the distro-specific configs, and only use os.environ['TZ'] and /etc/localtime on Linux.