Adding cython v0.20.2 in third-party.

third_party/cython/src/Cython/Compiler/DebugFlags.py

Index: third_party/cython/src/Cython/Compiler/DebugFlags.py
diff --git a/third_party/cython/src/Cython/Compiler/DebugFlags.py b/third_party/cython/src/Cython/Compiler/DebugFlags.py
new file mode 100644
index 0000000000000000000000000000000000000000..e830ab1849cf506ec10ab38ebd850a0a398c0431
--- /dev/null
+++ b/third_party/cython/src/Cython/Compiler/DebugFlags.py
@@ -0,0 +1,21 @@
+# Can be enabled at the command line with --debug-xxx.
+
+debug_disposal_code = 0
+debug_temp_alloc = 0
+debug_coercion = 0
+
+# Write comments into the C code that show where temporary variables
+# are allocated and released.
+debug_temp_code_comments = 0
+
+# Write a call trace of the code generation phase into the C code.
+debug_trace_code_generation = 0
+
+# Do not replace exceptions with user-friendly error messages.
+debug_no_exception_intercept = 0
+
+# Print a message each time a new stage in the pipeline is entered.
+debug_verbose_pipeline = 0
+
+# Raise an exception when an error is encountered.
+debug_exception_on_error = 0