Allow passing --no-upgrade option (#1438)

This makes this option similar with the other boolean ones and allows
calling the tools with explicit --no-upgrade.

This change ease integration with other tools like CI/CD pipelines
where user may want to change the default behavior related to
upgrades.

piptools/scripts/compile.py

@@ -125,7 +125,7 @@ def _get_default_option(option_name: str) -> Any:
 )
 @click.option(
     "-U",
-    "--upgrade",
+    "--upgrade/--no-upgrade",
     is_flag=True,
     default=False,
     help="Try to upgrade all dependencies to their latest versions",