Use Python 3 sematics in Cython-compiled modules.

sdks/python/apache_beam/coders/stream.pyx

@@ -15,6 +15,8 @@
 # limitations under the License.
 #
 
+# cython: language_level=3
+
 """Compiled version of the Stream objects used by CoderImpl.
 
 For internal use only; no backwards-compatibility guarantees.

sdks/python/apache_beam/metrics/cells.py

@@ -15,6 +15,8 @@
 # limitations under the License.
 #
 
+# cython: language_level=3
+
 """
 This file contains metric cell classes. A metric cell is used to accumulate
 in-memory changes to a metric. It represents a specific metric in a single

sdks/python/apache_beam/metrics/execution.py

@@ -15,6 +15,8 @@
 # limitations under the License.
 #
 
+# cython: language_level=3
+
 """
 This module is for internal use only; no backwards-compatibility guarantees.
 

sdks/python/apache_beam/runners/common.py

@@ -15,6 +15,7 @@
 # limitations under the License.
 #
 # cython: profile=True
+# cython: language_level=3
 
 """Worker operations executor.
 

sdks/python/apache_beam/runners/worker/statesampler_fast.pyx

@@ -16,6 +16,7 @@
 #
 
 # cython: profile=True
+# cython: language_level=3
 
 """State sampler for tracking time spent in execution steps.
 

sdks/python/apache_beam/transforms/cy_dataflow_distribution_counter.pyx

@@ -15,6 +15,7 @@
 #
 
 # cython: profile=True
+# cython: language_level=3
 
 """ For internal use only. No backwards compatibility guarantees."""
 

sdks/python/apache_beam/transforms/stats.py

@@ -15,6 +15,8 @@
 # limitations under the License.
 #
 
+# cython: language_level=3
+
 """This module has all statistic related transforms.
 
 This ApproximateUnique class will be deprecated [1]. PLease look into using

sdks/python/apache_beam/utils/counters.py

@@ -17,6 +17,7 @@
 
 # cython: profile=False
 # cython: overflowcheck=True
+# cython: language_level=3
 
 """Counters collect the progress of the Worker for reporting to the service.
 

sdks/python/apache_beam/utils/windowed_value.py

@@ -25,7 +25,8 @@
 # editing this file as WindowedValues are created for every element for
 # every step in a Beam pipeline.
 
-#cython: profile=True
+# cython: profile=True
+# cython: language_level=3
 
 # pytype: skip-file
 

sdks/python/setup.py

@@ -268,6 +268,7 @@ def run(self):
         '*/*.pyx', '*/*/*.pyx', '*/*.pxd', '*/*/*.pxd', '*/*.h', '*/*/*.h',
         'testing/data/*.yaml', 'portability/api/*.yaml']},
     ext_modules=cythonize([
+        # Make sure to use language_level=3 cython directive in files below.
         'apache_beam/**/*.pyx',
         'apache_beam/coders/coder_impl.py',
         'apache_beam/metrics/cells.py',