Fix type warning for invalid escape in string (#1003)

Fixes the following type checker warning:

```
mobly/controllers/attenuator_lib/telnet_scpi_client.py:73: SyntaxWarning: invalid escape sequence '\S'
  [_ascii_string("\S+" + self.rx_cmd_separator)], 1
```

mobly/controllers/attenuator_lib/telnet_scpi_client.py

diff --git a/mobly/controllers/attenuator_lib/telnet_scpi_client.py b/mobly/controllers/attenuator_lib/telnet_scpi_client.py
index 98cccca..b43f9e7 100644
--- a/mobly/controllers/attenuator_lib/telnet_scpi_client.py
+++ b/mobly/controllers/attenuator_lib/telnet_scpi_client.py
@@ -70,7 +70,7 @@
       return None
 
     match_idx, match_val, ret_text = self._tn.expect(
-        [_ascii_string("\S+" + self.rx_cmd_separator)], 1
+        [_ascii_string(r"\S+" + self.rx_cmd_separator)], 1
     )
 
     if match_idx == -1: