Note: this was causing a weird flake8 error on CI. Presumably due to CI running flake8 via py3.7.
```
flake8 . --count --select=$ELECTRUM_LINTERS --show-source --statistics
./electrum/scripts/update_default_servers.py:1:26: E999 SyntaxError: invalid syntax
#!/usr/bin/env python3
^
1 E999 SyntaxError: invalid syntax
1
```
Previously all the interfaces used either "t" or "s".
Now the network only tries to use "s" for all interfaces, except for
the main interface, which the user can manually specify to use "t".
(so e.g. if you run with "--server localhost:50002:t", the main server will use "t",
but all the rest will use "s")
The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)
