config: Change ConfigFileFinder._read_config() to accept variadic args

This simplifies `._read_config()` by removing a conditional branch in
the situation where it is called with one file to process.  Now the
contract accepts any number of arguments of the same type.

Where callers invoke `._read_config()` with a `Sequence`, the call site
has been changed to unpack arguments (i.e., `*`).

The tests in `test_merged_config_parser.py` needed to return a string
for the user configuration path instead of an empty list since
`ConfigFileFinder.user_config_file()` returns a string.