Handle missing optional dependencies in dep_versions()

PR #1159 made keyring optional on some (non-Desktop) architectures.
However the CLI attempts to determine dependency versions, in failure
codepaths. Rather than blow up entirely, report the dependency as
missing.

Part of: #1158

twine/cli.py

@@ -76,15 +76,24 @@ def configure_output() -> None:
 
 def list_dependencies_and_versions() -> List[Tuple[str, str]]:
     deps = [
-        "keyring",
+        "keyring",  # optional for non-desktop use
         "pkginfo",
         "requests",
         "requests-toolbelt",
         "urllib3",
     ]
     if sys.version_info < (3, 10):
         deps.append("importlib-metadata")
-    return [(dep, importlib_metadata.version(dep)) for dep in deps]
+
+    result: List[Tuple[str, str]] = []
+    for dep in deps:
+        try:
+            version = importlib_metadata.version(dep)
+        except importlib_metadata.PackageNotFoundError:
+            version = "NOT INSTALLED"
+        result.append((dep, version))
+
+    return result
 
 
 def dep_versions() -> str: