USDT is common many exchanges and treated as a currency.
It is helpful to allow USDT to be accepted as a fiat current when testing config.json
By way of example of a usecase
A strategy or otherwise report may not query: config['stake_currency'] and currency = config['fiat_display_currency']
as the config.json with fail test, so cannot build a base/pair such as BTC/USDT that is valid will not be built cleanly.
Pulling BTC/USDT from config values can be useful to download query ticker file name and data, enumerating base/pair price to whitelist target pairs, performing correlation coefficient statistics etc.
As example: building a dataframe such as here
date BTC/USDT ETH/BTC XLM/BTC LTC/BTC
0 1517443200 10283.00 0.109475 0.000052 0.016031
The transposing to actual USDT ticker value row-wise per pair
date BTC/USDT ETH/USD XLM/USD LTC/USD
1517443200 10283.00 1125.731425 0.535950 164.846773
