Add automated tests for contents

Since apparently this was silently failing, it's probably best to
actually add some tests here.

There are two testing strategies here:

1. Test that every file in the zones list (and the zones list itself) is
   included in the package and contains a TZif file.

2. Test a smattering of hard-coded zones - this is in case the zones
   list populates incorrectly for some reason (e.g. an empty list). I
   didn't want to exhaustively list all the zones because that seems too
   much to maintain, but I figure some hard-coded zones from each of the
   general areas should be a good canary in the coal mine.

Author: pganssle

tests/test_contents.py

@@ -0,0 +1,67 @@
+try:
+    from importlib import resources
+except ImportError:
+    import importlib_resources as resources
+
+import pytest
+
+
+def get_magic(zone_name):
+    components = zone_name.split("/")
+    package_name = ".".join(["tzdata.zoneinfo"] + components[:-1])
+    resource_name = components[-1]
+
+    with resources.open_binary(package_name, resource_name) as f:
+        return f.read(4)
+
+
+@pytest.mark.parametrize(
+    "zone_name",
+    [
+        "Africa/Cairo",
+        "Africa/Casablanca",
+        "Africa/Lome",
+        "America/Argentina/San_Luis",
+        "America/Denver",
+        "America/Los_Angeles",
+        "America/New_York",
+        "America/Thunder_Bay",
+        "Antarctica/South_Pole",
+        "Asia/Calcutta",
+        "Asia/Damascus",
+        "Asia/Seoul",
+        "Atlantic/Reykjavik",
+        "Australia/Perth",
+        "Egypt",
+        "Etc/GMT-9",
+        "Europe/Dublin",
+        "Europe/London",
+        "Europe/Prague",
+        "Hongkong",
+        "Indian/Cocos",
+        "Indian/Mayotte",
+        "Mexico/BajaNorte",
+        "Pacific/Guam",
+        "Pacific/Kiritimati",
+        "US/Eastern",
+        "UTC",
+    ],
+)
+def test_zone_valid(zone_name):
+    """Test an assortment of hard-coded zone names.
+
+    This test checks that the zone resource can be loaded and that it starts
+    with the 4-byte magic indicating a TZif file.
+    """
+    magic = get_magic(zone_name)
+    assert magic == b"TZif"
+
+
+def test_load_zones(subtests):
+    with resources.open_text("tzdata", "zones") as f:
+        zones = [z.strip() for z in f]
+
+    for zone in zones:
+        with subtests.test(zone=zone):
+            magic = get_magic(zone)
+            assert magic == b"TZif"

tox.ini

@@ -3,6 +3,15 @@ minversion = 3.3.0
 isolated_build = True
 skip_missing_interpreters = true
 
+[testenv]
+description = Test that the tzdata contents are accessible
+deps =
+    pytest
+    pytest-subtests
+    importlib_resources; python_version<'3.7'
+commands =
+    pytest {toxinidir} {posargs}
+
 [testenv:update]
 description = Update the tzdata contents
 skip_install = True
@@ -30,7 +39,7 @@ deps =
 commands =
     black .
     isort update.py
-
+    isort --atomic -rc tests
 
 [testenv:build]
 description = Build a wheel and source distribution