Add sftp=None support for creating SFTP channel to SSHClient.mkdir()

[snooter101]

Is your feature request related to a problem? Please describe.
First try at using parallel-ssh and am trying to create a remote directory using SSHClient.mkdir().

The method requires the sftp parameter and doesn't allow None, while other similar methods (e.g. SSHClient.copy_file()) do.

I looked at the source and worked out what I needed to do (i.e. use the internal method _make_sftp()), but it would be nicer for it to be created on the fly.

Describe the solution you'd like
Default sftp=None for SSHClient.mkdir(), and when sftp=None, then use _make_sftp() to create the SFTP channel.

I realise this changes the API so might be something that needs to be changed in a later version.
Also, adding the same default sftp handling to other methods like sftp_get() and sftp_put() would simplify usage as well.

Describe alternatives you've considered
Used internal method to create SFTP channel.

Additional context
None.