Run black at end of synth.py (#6698)

* Run black at end of synth.py
* blacken logging

Author: crwilcox

packages/google-cloud-spanner/synth.py

@@ -130,3 +130,5 @@
 # ----------------------------------------------------------------------------
 templated_files = common.py_library(unit_cov_level=97, cov_level=100)
 s.move(templated_files)
+
+s.shell.run(["nox", "-s", "blacken"], hide_output=False)