Use brackets for the default value of option arguments

The goal is to standardize the format of the help text printed
by pypa commands. It is not easy to choose between brackets `[]`
and parentheses `()`. I went for the docopt style, which is the
closest to a standard I could find:
	http://docopt.org/

	[...] and whether that argument has a default value ([default: 10]).

This change has already been applied to setuptools:
	pypa/setuptools#4442

twine/settings.py

@@ -165,8 +165,8 @@ def register_argparse_arguments(parser: argparse.ArgumentParser) -> None:
             env="TWINE_REPOSITORY",
             default="pypi",
             help="The repository (package index) to upload the package to. "
-            "Should be a section in the config file (default: "
-            "%(default)s). (Can also be set via %(env)s environment "
+            "Should be a section in the config file [default: "
+            "%(default)s]. (Can also be set via %(env)s environment "
             "variable.)",
         )
         parser.add_argument(
@@ -195,7 +195,7 @@ def register_argparse_arguments(parser: argparse.ArgumentParser) -> None:
         parser.add_argument(
             "--sign-with",
             default="gpg",
-            help="GPG program used to sign uploads (default: %(default)s).",
+            help="GPG program used to sign uploads [default: %(default)s].",
         )
         parser.add_argument(
             "-i",