Fixed test failures on MySQL.

Some tests failed when the time zone definitions were loaded in MySQL
and pytz wasn't installed. This setup isn't supported.

django/db/backends/mysql/base.py

@@ -30,6 +30,11 @@ if (version < (1, 2, 1) or (version[:3] == (1, 2, 1) and
 from MySQLdb.converters import conversions, Thing2Literal
 from MySQLdb.constants import FIELD_TYPE, CLIENT
 
+try:
+    import pytz
+except ImportError:
+    pytz = None
+
 from django.conf import settings
 from django.db import utils
 from django.db.backends import *
@@ -186,6 +191,15 @@ class DatabaseFeatures(BaseDatabaseFeatures):
 
     @cached_property
     def has_zoneinfo_database(self):
+        # MySQL accepts full time zones names (eg. Africa/Nairobi) but rejects
+        # abbreviations (eg. EAT). When pytz isn't installed and the current
+        # time zone is LocalTimezone (the only sensible value in this
+        # context), the current time zone name will be an abbreviation. As a
+        # consequence, MySQL cannot perform time zone conversions reliably.
+        if pytz is None:
+            return False
+
+        # Test if the time zone definitions are installed.
         cursor = self.connection.cursor()
         cursor.execute("SELECT 1 FROM mysql.time_zone LIMIT 1")
         return cursor.fetchone() is not None