localtz.patch

The CPython's implementation of `datetime.fromtimestamp()`,
`datetime.astimezone()` and `datetime.timestamp()` for naive datetime objects
relay on proper management of DST (daylight saving time) by `time.localtime()`
for the timezone of interest. In the Unix port of MicroPython, this is
accomplished by properly setting the TZ environment variable, e.g.
`os.putenv("TZ", "Europe/Rome")`.

Because real boards often lack a supportive `time.localtime()`, the source code
in `datetime.py` has been removed as to save precious resources. If your board
provide a proper implementation, you can restore the support to naive datetime
objects by applying this patch, e.g. `patch -p1 < localtz.patch`.

--- a/datetime.py
+++ b/datetime.py
@@ -635,7 +635,10 @@ class datetime:
         else:
             us = 0
         if tz is None:
-            raise NotImplementedError
+            dt = cls(*_tmod.localtime(ts)[:6], microsecond=us, tzinfo=tz)
+            s = (dt - datetime(*_tmod.localtime(ts - 86400)[:6]))._us // 1_000_000 - 86400
+            if s < 0 and dt == datetime(*_tmod.localtime(ts + s)[:6]):
+                dt._fd = 1
         else:
             dt = cls(*_tmod.gmtime(ts)[:6], microsecond=us, tzinfo=tz)
             dt = tz.fromutc(dt)
@@ -812,13 +815,45 @@ class datetime:
             return self
         _tz = self._tz
         if _tz is None:
-            raise NotImplementedError
+            ts = int(self._mktime())
+            os = datetime(*_tmod.localtime(ts)[:6]) - datetime(*_tmod.gmtime(ts)[:6])
         else:
             os = _tz.utcoffset(self)
         utc = self - os
         utc = utc.replace(tzinfo=tz)
         return tz.fromutc(utc)
 
+    def _mktime(self):
+        def local(u):
+            return (datetime(*_tmod.localtime(u)[:6]) - epoch)._us // 1_000_000
+
+        epoch = datetime.EPOCH.replace(tzinfo=None)
+        t, us = divmod((self - epoch)._us, 1_000_000)
+        ts = None
+
+        a = local(t) - t
+        u1 = t - a
+        t1 = local(u1)
+        if t1 == t:
+            u2 = u1 + (86400 if self.fold else -86400)
+            b = local(u2) - u2
+            if a == b:
+                ts = u1
+        else:
+            b = t1 - u1
+        if ts is None:
+            u2 = t - b
+            t2 = local(u2)
+            if t2 == t:
+                ts = u2
+            elif t1 == t:
+                ts = u1
+            elif self.fold:
+                ts = min(u1, u2)
+            else:
+                ts = max(u1, u2)
+        return ts + us / 1_000_000
+
     def utcoffset(self):
         return None if self._tz is None else self._tz.utcoffset(self)
 
@@ -842,7 +877,7 @@ class datetime:
 
     def timestamp(self):
         if self._tz is None:
-            raise NotImplementedError
+            return self._mktime()
         else:
             return (self - datetime.EPOCH).total_seconds()