Fix type warning for invalid escape in string

[mbrukman]

This change will fix 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

[xpconanfan]

This seems to break presubmit tests on Windows

[mbrukman]

@xpconanfan — the problem with that build is that installation of Python 3.11 is failing on Windows:

C:\tmpfs\src\github\mobly>choco install python311
Chocolatey v2.5.1
Installing the following packages:
python311
By installing, you accept licenses for the packages.
Unable to connect to source 'https://community.chocolatey.org/api/v2/':
Failed to fetch results from V2 feed at 'https://community.chocolatey.org/api/v2/Packages()?$filter=(tolower(Id)%20eq%20'python311')%20and%20IsLatestVersion&semVerLevel=2.0.0' with following message : Response status code does not indicate success: 499 ().
python311 not installed. The package was not found with the source(s) listed.

Error code 499 seems to be due to the client closing the connection while the server was still processing the request, so it may be worthwhile to just rerun that build to see if it succeeds on the next run.

I'll re-push the branch to re-trigger the build to see if that fixes it. If it doesn't, I will debug further.

[mbrukman]

@xpconanfan — all tests passed upon rerun.