Fix type of sshexecutor stdin parameter

[giffels]

During migration of our active C/T instances to a different host, we are using the first time the HTCondorSiteAdapter in combination with the SSHExecutor. In addition, the HTCondorSiteAdapter is the only adapter that uses the stdin to add input to the condor_submit.

That triggered a bug in the SSHExecutor, which assumed that asyncssh requires stdin to be in bytes format. However, according to the documentation, it is required to be in string format.

This pull request changes the type of SSHExecutor from byte format to string format.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (36ac7c1) 98.86% compared to head (5bc20d4) 98.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #314 +/- ## ======================================= Coverage 98.86% 98.86% ======================================= Files 55 55 Lines 2210 2210 ======================================= Hits 2185 2185 Misses 25 25 ``` | [Files](https://app.codecov.io/gh/MatterMiners/tardis/pull/314?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MatterMiners) | Coverage Δ | | |---|---|---| | [tardis/utilities/executors/sshexecutor.py](https://app.codecov.io/gh/MatterMiners/tardis/pull/314?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MatterMiners#diff-dGFyZGlzL3V0aWxpdGllcy9leGVjdXRvcnMvc3NoZXhlY3V0b3IucHk=) | `100.00% <ø> (ø)` | |

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