Add env var for --non-interactive flag

Change --non-interactive action to utils.EnvironmentDefault to support specification as an env var

README.rst

@@ -208,7 +208,9 @@ Uploads one or more distributions to a repository.
                             (package index) with. (Can also be set via
                             TWINE_PASSWORD environment variable.)
       --non-interactive     Do not interactively prompt for username/password
-                            if the required credentials are missing.
+                            if the required credentials are missing. (Can also
+                            be set via TWINE_NON_INTERACTIVE environment
+                            variable.)
       -c COMMENT, --comment COMMENT
                             The comment to include with the distribution file.
       --config-file CONFIG_FILE
@@ -288,7 +290,9 @@ For completeness, its usage:
                             (package index) with. (Can also be set via
                             TWINE_PASSWORD environment variable.)
       --non-interactive     Do not interactively prompt for username/password
-                            if the required credentials are missing.
+                            if the required credentials are missing. (Can also
+                            be set via TWINE_NON_INTERACTIVE environment
+                            variable.)
       -c COMMENT, --comment COMMENT
                             The comment to include with the distribution file.
       --config-file CONFIG_FILE
@@ -315,6 +319,8 @@ example.
 * ``TWINE_REPOSITORY_URL`` - the repository URL to use.
 * ``TWINE_CERT`` - custom CA certificate to use for repositories with
   self-signed or untrusted certificates.
+* ``TWINE_NON_INTERACTIVE`` - Do not interactively prompt for username/password
+  if the required credentials are missing.
 
 Resources
 ---------

twine/settings.py

@@ -201,11 +201,13 @@ def register_argparse_arguments(parser: argparse.ArgumentParser) -> None:
         )
         parser.add_argument(
             "--non-interactive",
-            action="store_true",
+            action=utils.EnvironmentDefault,
+            env="TWINE_NON_INTERACTIVE",
             default=False,
             required=False,
             help="Do not interactively prompt for username/password if the "
-                 "required credentials are missing."
+                 "required credentials are missing. (Can also be set via "
+                 "%(env)s environment variable.)"
         )
         parser.add_argument(
             "-c", "--comment",