Revert "Migrate as much as possible to pip and requirements.txt"

This reverts commit 982594a4a5.

lib/pytz/exceptions.py

@@ -0,0 +1,48 @@
+'''
+Custom exceptions raised by pytz.
+'''
+
+__all__ = [
+    'UnknownTimeZoneError', 'InvalidTimeError', 'AmbiguousTimeError',
+    'NonExistentTimeError',
+    ]
+
+
+class UnknownTimeZoneError(KeyError):
+    '''Exception raised when pytz is passed an unknown timezone.
+
+    >>> isinstance(UnknownTimeZoneError(), LookupError)
+    True
+
+    This class is actually a subclass of KeyError to provide backwards
+    compatibility with code relying on the undocumented behavior of earlier
+    pytz releases.
+
+    >>> isinstance(UnknownTimeZoneError(), KeyError)
+    True
+    '''
+    pass
+
+
+class InvalidTimeError(Exception):
+    '''Base class for invalid time exceptions.'''
+
+
+class AmbiguousTimeError(InvalidTimeError):
+    '''Exception raised when attempting to create an ambiguous wallclock time.
+
+    At the end of a DST transition period, a particular wallclock time will
+    occur twice (once before the clocks are set back, once after). Both
+    possibilities may be correct, unless further information is supplied.
+
+    See DstTzInfo.normalize() for more info
+    '''
+
+
+class NonExistentTimeError(InvalidTimeError):
+    '''Exception raised when attempting to create a wallclock time that
+    cannot exist.
+
+    At the start of a DST transition period, the wallclock time jumps forward.
+    The instants jumped over never occur.
+    '''