Use brackets for the default value of option arguments

The goal is to standardize the format of the help text printed
by 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 distutils:
	pypa/distutils#262

src/wheel/bdist_wheel.py

@@ -179,7 +179,7 @@ class bdist_wheel(Command):
             "plat-name=",
             "p",
             "platform name to embed in generated filenames "
-            f"(default: {get_platform(None)})",
+            f"[default: {get_platform(None)}]",
         ),
         (
             "keep-temp",
@@ -192,7 +192,7 @@ class bdist_wheel(Command):
         (
             "relative",
             None,
-            "build the archive using relative paths (default: false)",
+            "build the archive using relative paths [default: false]",
         ),
         (
             "owner=",
@@ -204,18 +204,18 @@ class bdist_wheel(Command):
             "g",
             "Group name used when creating a tar file [default: current group]",
         ),
-        ("universal", None, "make a universal wheel (default: false)"),
+        ("universal", None, "make a universal wheel [default: false]"),
         (
             "compression=",
             None,
-            "zipfile compression (one of: {}) (default: 'deflated')".format(
+            "zipfile compression (one of: {}) [default: 'deflated']".format(
                 ", ".join(supported_compressions)
             ),
         ),
         (
             "python-tag=",
             None,
-            f"Python implementation compatibility tag (default: '{python_tag()}')",
+            f"Python implementation compatibility tag [default: '{python_tag()}']",
         ),
         (
             "build-number=",
@@ -227,7 +227,7 @@ class bdist_wheel(Command):
         (
             "py-limited-api=",
             None,
-            "Python tag (cp32|cp33|cpNN) for abi3 wheel tag (default: false)",
+            "Python tag (cp32|cp33|cpNN) for abi3 wheel tag [default: false]",
         ),
     ]