Spur

* Raise minimum Python version to 3.6.

* Handle FileNotFoundError when using subprocess on Python >= 3.8.

* Fix: default connect timeout was not being set correctly.

* Add close() method to shells, behaving the same as __exit__().

* SshShell: passing None as the port argument will cause port 22 to be used.

* Raise CouldNotChangeDirectoryError when failing to change directory to the cwd
argument.

* Allow spur to be used with Paramiko 2. To quote the Paramiko changelog [1]:

Replace PyCrypto with the Python Cryptographic Authority (PyCA)
‘Cryptography’ library suite. This improves security, installability, and
performance; adds PyPy support; and much more.

This should only affect installation requirements for Paramiko: see the
changelog for more information. Since the API of Paramiko remains unchanged,
spur should remain compatible with both Paramiko 1 and Paramiko 2.

Note that Python 3.2 is not compatible with Paramiko 2: you can continue to
use spur with Python 3.2 and Paramiko 1, although upgrading is recommended.

[1]: http://www.paramiko.org/changelog.html#2.0.0

* Add sock argument to SshShell.