check-python-versions --add ... drops the Python :: 3 :: Only classifier

[mgedmin]

Steps to reproduce:

1. git clone https://github.com/mgedmin/check-manifest
2. git checkout 61023f599d5bbdb2b5b7ff2e9404f0e40ca170f7
3. check-python-versions --add 3.10 --diff

Expected:

• adds 3.10, doesn't touch anything else

Actual:

--- ./setup.py	(original)
+++ ./setup.py	(updated)
@@ -44,11 +44,11 @@
         'Operating System :: OS Independent',
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3 :: Only',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
+        'Programming Language :: Python :: 3.10',
         'Programming Language :: Python :: Implementation :: CPython',
         'Programming Language :: Python :: Implementation :: PyPy',
     ],