Deprecate reading configuration from `.my.cnf`

[rolandwalker]

As mentioned in #581, it has been a longstanding desire to migrate away from reading .my.cnf in favor of taking configuration only from ~/.myclirc.

The reasons in short are: either

• a given config option exists only in one file or the other, which creates a burden on the user to know which one, or
• a given config option exists in both, which creates a burden on the user to understand which overrides.

In addition, supporting two configuration files adds code complexity.

We can begin with warnings, which will take some work to implement, but are possible, then run a lengthy deprecation cycle as in #1464 .

It may be that we have to provide a migration tool, either built in to mycli, or as a standalone tool.

[rolandwalker]

It may be that we have to provide a migration tool, either built in to mycli, or as a standalone tool.

In #1493 we take the approach of a verbose warning with instructions to edit ~/.myclirc, rather than a migration tool.