Update development dependency twine to v3.2.0

Twine version < 2.0 was reported has a security vulnerability

From Twine version 2.0+, it requires Python 3.6, but doesn't seem to
have other breaking changes.

Reference:

- pypa/twine#491
- https://twine.readthedocs.io/en/latest/changelog.html

requirements-dev.txt

@@ -5,4 +5,4 @@ pycodestyle==2.6.0
 pyflakes==2.2.0
 readme_renderer[md]==26.0
 requests_mock
-twine==1.15.0
+twine==3.2.0