Redo the synonym handling completely, and now only use zoneinfo symlinks

- It turns out a lot of Linux distributions make the links between zoneinfo
  aliases backwards, so instead of linking GB to Europe/London it actually
  links the other way. When /etc/localtime then links to Europe/London, and you
  also have a config file saying Europe/London, the code that checks if
  /etc/localtime is a symlink ends up at GB instead of Europe/London and
  we get an error, as it thinks GB and Europe/London are different zones.

  So now we check the symlink of all timezones in the uniqueness test. We still
  return the name in the config file, though, so you would only get GB or Zulu
  returned as the time zone instead of Europe/London or UTC if your only
  configuration is the /etc/localtime symlink, as that's checked last, and
  tzlocal will return the first configuration found.

- The above change also means that GMT and UTC are no longer seen as synonyms,
  as zoneinfo does not see them as synonyms. This might be controversial,
  but you just have to live with it. Pick one and stay with it. ;-)

CHANGES.txt

@@ -4,7 +4,22 @@ Changes
 4.0.3 (unreleased)
 ------------------
 
-- Nothing changed yet.
+- It turns out a lot of Linux distributions make the links between zoneinfo
+  aliases backwards, so instead of linking GB to Europe/London it actually
+  links the other way. When /etc/localtime then links to Europe/London, and you
+  also have a config file saying Europe/London, the code that checks if
+  /etc/localtime is a symlink ends up at GB instead of Europe/London and
+  we get an error, as it thinks GB and Europe/London are different zones.
+
+  So now we check the symlink of all timezones in the uniqueness test. We still
+  return the name in the config file, though, so you would only get GB or Zulu
+  returned as the time zone instead of Europe/London or UTC if your only
+  configuration is the /etc/localtime symlink, as that's checked last, and
+  tzlocal will return the first configuration found.
+
+- The above change also means that GMT and UTC are no longer seen as synonyms,
+  as zoneinfo does not see them as synonyms. This might be controversial,
+  but you just have to live with it. Pick one and stay with it. ;-)
 
 
 4.0.2 (2021-10-26)

tests/test_data/noconflict/etc/localtime

@@ -0,0 +1 @@
+../usr/share/zoneinfo/Zulu

tests/test_data/noconflict/etc/timezone

@@ -1 +1 @@
-UTC# We allow comments. It's unusual, but has happened
+Etc/UTC# We allow comments. It's unusual, but has happened

tests/test_data/noconflict/usr/share/zoneinfo/Etc/UCT

@@ -0,0 +1 @@
+UTC

tests/test_data/noconflict/usr/share/zoneinfo/UTC

@@ -0,0 +1 @@
+Etc/UCT

tests/test_data/noconflict/usr/share/zoneinfo/Zulu

@@ -0,0 +1 @@
+UTC

tests/test_tzlocal.py

@@ -271,9 +271,12 @@ def test_conflicting():
     assert "localtime is a symlink to: Africa/Harare" in message
 
 
+@pytest.mark.skipif(
+    platform.system() == "Windows", reason="Symbolic links are not available on Windows"
+)
 def test_noconflict():
     tz = tzlocal.unix._get_localzone(_root=tz_path("noconflict"))
-    assert str(tz) == "UTC"
+    assert str(tz) == "Etc/UTC"
 
 
 def test_pytz_compatibility():

tzlocal/unix.py

@@ -126,18 +126,15 @@ def _get_localzone_name(_root="/"):
         if len(found_configs) > 1:
             # Uh-oh, multiple configs. See if they match:
             unique_tzs = set()
+            zoneinfo = os.path.join(_root, "usr", "share", "zoneinfo")
+            directory_depth = len(zoneinfo.split(os.path.sep))
+
             for tzname in found_configs.values():
-                # Get rid of any Etc's
-                tzname = tzname.replace("Etc/", "")
-                # Let's handle these synonyms as well. Many systems have tons
-                # of synonyms, including country names and other non-zoneinfo
-                # nonsense. Those will be seen as different ones. Let's stick
-                # to the official zoneinfo Continent/City names.
-                if tzname in ["GMT0", "GMT+0", "GMT-0"]:
-                    tzname = "GMT"
-                if tzname in ["UCT", "Zulu"]:
-                    tzname = "UTC"
-                unique_tzs.add(tzname)
+                # Look them up in /usr/share/zoneinfo, and find what they
+                # really point to:
+                path = os.path.realpath(os.path.join(zoneinfo, *tzname.split("/")))
+                real_zone_name = "/".join(path.split(os.path.sep)[directory_depth:])
+                unique_tzs.add(real_zone_name)
 
             if len(unique_tzs) != 1:
                 message = "Multiple conflicting time zone configurations found:\n"