Use Python 3 in release scripts

# ...
06:17:02  + tools/release.sh --publish alpha
# ...
06:17:17  >>> [alpha 4.4.4.147] Generating 4.4.4.147.yml
06:17:18  Collecting pyyaml==5.1.2
06:17:18  Installing collected packages: pyyaml
06:17:18  Exception:
06:17:18  Traceback (most recent call last):
06:17:18    File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 215, in main
06:17:18      status = self.run(options, args)
06:17:18    File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 360, in run
06:17:18      prefix=options.prefix_path,
06:17:18    File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 784, in install
06:17:18      **kwargs
06:17:18    File "/usr/lib/python2.7/dist-packages/pip/req/req_install.py", line 851, in install
06:17:18      self.move_wheel_files(self.source_dir, root=root, prefix=prefix)
06:17:18    File "/usr/lib/python2.7/dist-packages/pip/req/req_install.py", line 1064, in move_wheel_files
06:17:18      isolated=self.isolated,
06:17:18    File "/usr/lib/python2.7/dist-packages/pip/wheel.py", line 247, in move_wheel_files
06:17:18      prefix=prefix,
06:17:18    File "/usr/lib/python2.7/dist-packages/pip/locations.py", line 141, in distutils_scheme
06:17:18      d.parse_config_files()
06:17:18    File "/usr/lib/python2.7/distutils/dist.py", line 397, in parse_config_files
06:17:18      val = parser.get(section,opt)
06:17:18    File "/usr/lib/python2.7/ConfigParser.py", line 623, in get
06:17:18      return self._interpolate(section, option, value, d)
06:17:18    File "/usr/lib/python2.7/ConfigParser.py", line 669, in _interpolate
06:17:18      option, section, rawval, e.args[0])
06:17:18  InterpolationMissingOptionError: Bad value substitution:
06:17:18  	section: [tool:pytest]
06:17:18  	option : log_format
06:17:18  	key    : asctime
06:17:18  	rawval : %%(asctime)s %%(process)d %%(thread)d %%(levelname)-8s %%(name)-18s %%(message)s
# ...