feat: use contextvars.ConvextVar instead of threading.local in Python 3

Closes #504 

Co-authored-by: ndavydovdev <ndavydov@aita.travel>

google/cloud/ndb/context.py

@@ -26,12 +26,43 @@
 from google.cloud.ndb import tasklets
 
 
-class _LocalState(threading.local):
-    """Thread local state."""
+try:  # pragma: NO PY2 COVER
+    import contextvars
 
-    def __init__(self):
-        self.context = None
-        self.toplevel_context = None
+    class _LocalState:
+        """Thread local state."""
+
+        def __init__(self):
+            self._toplevel_context = contextvars.ContextVar(
+                "_toplevel_context", default=None
+            )
+            self._context = contextvars.ContextVar("_context", default=None)
+
+        @property
+        def context(self):
+            return self._context.get()
+
+        @context.setter
+        def context(self, value):
+            self._context.set(value)
+
+        @property
+        def toplevel_context(self):
+            return self._toplevel_context.get()
+
+        @toplevel_context.setter
+        def toplevel_context(self, value):
+            self._toplevel_context.set(value)
+
+
+except ImportError:  # pragma: NO PY3 COVER
+
+    class _LocalState(threading.local):
+        """Thread local state."""
+
+        def __init__(self):
+            self.context = None
+            self.toplevel_context = None
 
 
 _state = _LocalState()

setup.py

@@ -14,6 +14,7 @@
 
 import io
 import os
+import sys
 
 import setuptools
 
@@ -28,6 +29,9 @@ def main():
         "redis",
     ]
 
+    if sys.version_info.major == 3 and sys.version_info.minor < 7:
+        dependencies.append("contextvars")
+
     setuptools.setup(
         name="google-cloud-ndb",
         version = "1.4.2",