Avoid Maintenance Sites in 'get_random_sites'

[paul-ruth]

Title says it all

[sajith]

Just came across a failure when get_random_site() chose a site in maintenance while reviewing PR #126 and working through "Using FABRIC NVMe Devices" notebook:

Exception: Failed to submit slice: Status.FAILURE, (500)
Reason: INTERNAL SERVER ERROR
HTTP response headers: HTTPHeaderDict({'Server': 'nginx/1.21.6', 'Date': 'Tue, 21 Feb 2023 17:27:31 GMT', 'Content-Type': 'text/html; charset=utf-8', 'Content-Length': '194', 'Connection': 'keep-alive', 'Access-Control-Allow-Credentials': 'true', 'Access-Control-Allow-Headers': 'DNT, User-Agent, X-Requested-With, If-Modified-Since, Cache-Control, Content-Type, Range, Authorization', 'Access-Control-Allow-Methods': 'GET, POST, PUT, PATCH, DELETE, OPTIONS', 'Access-Control-Allow-Origin': '*', 'Access-Control-Expose-Headers': 'Content-Length, Content-Range, X-Error', 'X-Error': 'Site GATECH in Maint'})
HTTP response body: b'{\n    "errors": [\n        {\n            "details": "Site GATECH in Maint",\n            "message": "Internal Server Error"\n        }\n    ],\n    "size": 1,\n    "status": 500,\n    "type": "error"\n}'

Just wanted to document the error here for future reference.