Added DandelionPhase to pick stem mode or not

src/gossip/phase.py

@@ -0,0 +1,33 @@
+from time import time
+from hashlib import shake_128
+
+
+class DandelionPhase:
+    def __init__(self, seed: bytes, epoch_interval_secs: int):
+        self.seed = seed  # Seed intended to be from good random source like urandom
+        assert len(self.seed) == 32
+        self.epoch = int(time())
+        self.epoch_interval = epoch_interval_secs
+        self._is_stem = True
+
+
+    def _update_stem_phase(self, cur_time):
+        self.epoch = cur_time
+        # Hash the seed with the time stamp to produce 1 pseudorandom byte
+        # We produce an len(8) byte string for year 2038 problem
+        choice = shake_128(
+            self.seed +
+            int.to_bytes(cur_time, 8, 'big')).digest(1)
+
+        # If the byte is even
+        if int.from_bytes(choice, 'big') % 2:
+            self._is_stem = True
+        else:
+            self._is_stem = False
+
+
+    def is_stem_phase(self) -> bool:
+        current_time = int(time())
+        if current_time - self.epoch >= self.epoch_interval:
+            self._update_stem_phase(current_time)
+        return self._is_stem