pssh is output RuntimeWarning when using Python3.8

[junya-yamashita]

When using pssh commands with -e or -o options,I received below warning.

```
/usr/lib/python3/dist-packages/psshlib/manager.py:304: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.files[filename] = open(filename, 'wb', buffering=1)
```

example:

```
vagrant@ubuntu-2004:~$ /usr/bin/parallel-ssh -H vagrant@127.0.0.1 -e stderr -i 'hostname'
/usr/lib/python3/dist-packages/psshlib/manager.py:304: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  self.files[filename] = open(filename, 'wb', buffering=1)
[1] 15:19:59 [SUCCESS] vagrant@127.0.0.1
ubuntu-2004
```

It seems can fix /usr/lib/python3/dist-packages/psshlib/manager.py:304 as below.

```
+ self.files[filename] = open(filename, 'wb', buffering=0)
- self.files[filename] = open(filename, 'wb', buffering=1)
```

please fix package.Thank you.

Reference:https://bugs.python.org/issue32236
Environment:

```
vagrant@ubuntu-2004:~$ python3 --version
Python 3.8.5
vagrant@ubuntu-2004:~$ /usr/bin/parallel-ssh --version
2.3.1
vagrant@ubuntu-2004:~$ cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.1 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.1 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
```

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.