Close the session once snap connections are acquired (#8438)

This PR uses the context manager available for `requests.Session` to close properly the `session` once snap connections have been acquired.

certbot/certbot/_internal/snap_config.py

@@ -49,7 +49,7 @@ def prepare_env(cli_args):
     os.environ['CERTBOT_AUGEAS_PATH'] = '{0}/usr/lib/{1}/libaugeas.so.0'.format(
         os.environ.get('SNAP'), _ARCH_TRIPLET_MAP[snap_arch])
 
-    session = Session()
+    with Session() as session:
         session.mount('http://snapd/', _SnapdAdapter())
 
         try: