datetime: Make "fold" attribute handling optional

Python's datetime objects have a "fold" attribute since Python 3.6,
which was properly forwarded when converting between Python's datetime
and Hamster's datetime. However, this prevented to code from working
with Python versions below 3.6.

This commit makes handling of the fold attribute optional (e.g. it is
only used when present), which makes this code work on older Python
versions again.

src/hamster/lib/datetime.py

@@ -139,12 +139,12 @@ class time(pdt.time):
     def __new__(cls,
                 hour=0, minute=0,
                 second=0, microsecond=0,
-                tzinfo=None, fold=0):
+                tzinfo=None, **kwargs):
             # round down to zero seconds and microseconds
             return pdt.time.__new__(cls,
                                     hour=hour, minute=minute,
                                     second=0, microsecond=0,
-                                    tzinfo=None, fold=fold)
+                                    tzinfo=None, **kwargs)
 
     @classmethod
     def _extract_time(cls, match, h="hour", m="minute"):
@@ -220,12 +220,12 @@ class datetime(pdt.datetime):
     def __new__(cls, year, month, day,
                 hour=0, minute=0,
                 second=0, microsecond=0,
-                tzinfo=None, *, fold=0):
+                tzinfo=None, **kwargs):
             # round down to zero seconds and microseconds
             return pdt.datetime.__new__(cls, year, month, day,
                                         hour=hour, minute=minute,
                                         second=0, microsecond=0,
-                                        tzinfo=None, fold=fold)
+                                        tzinfo=None, **kwargs)
 
     def __add__(self, other):
         # python datetime.__add__ was not type stable prior to 3.8
@@ -234,11 +234,13 @@ def __add__(self, other):
     # similar to https://stackoverflow.com/q/51966126/3565696
     # __getnewargs_ex__ did not work, brute force required
     def __deepcopy__(self, memo):
+        kwargs = {}
+        if (hasattr(self, 'fold')):
+            kwargs['fold'] = self.fold
         return datetime(self.year, self.month, self.day,
                         self.hour, self.minute,
                         self.second, self.microsecond,
-                        self.tzinfo, fold=self.fold)
-
+                        self.tzinfo, **kwargs)
     __radd__ = __add__
 
     def __sub__(self, other):
@@ -327,10 +329,13 @@ def from_day_time(cls, d: hday, t: time):
     @classmethod
     def from_pdt(cls, t):
         """Convert python datetime to hamster datetime."""
+        kwargs = {}
+        if (hasattr(t, 'fold')):
+            kwargs['fold'] = t.fold
         return cls(t.year, t.month, t.day,
                    t.hour, t.minute,
                    t.second, t.microsecond,
-                   t.tzinfo, fold=t.fold)
+                   t.tzinfo, **kwargs)
 
     @classmethod
     def now(cls):
@@ -392,10 +397,13 @@ def pattern(cls, n=None):
 
     def to_pdt(self):
         """Convert to python datetime."""
+        kwargs = {}
+        if (hasattr(self, 'fold')):
+            kwargs['fold'] = self.fold
         return pdt.datetime(self.year, self.month, self.day,
                             self.hour, self.minute,
                             self.second, self.microsecond,
-                            self.tzinfo, fold=self.fold)
+                            self.tzinfo, **kwargs)
 
 
 # outside class; need the class to be defined first