`pidcat` installed via `brew` no longer works on macOS 12.3, due to Python 2 having been removed

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Hey!

First off, thank you for the great util, Jake. And all the other cool stuff you've built througout the years.

I'm having an issue where `pidcat`, installed from `brew`, can no longer run on macOS 12.3. I believe this is because of Python 2 having been removed on this version of macOS.

I'm getting the following output, when attempting to run `pidcat`:

```sh
~ pidcat
zsh: /opt/homebrew/bin/pidcat: bad interpreter: /usr/bin/python: no such file or directory
```

This in turn is caused by https://github.com/JakeWharton/pidcat/blob/44b4a106bb25776cbb551044b303520a07a51c1e/pidcat.py#L1:

```sh
#!/usr/bin/python -u
```

On macOS 12.3, `/usr/bin/python` does not exist - only `/usr/bin/python3`.

Running on the latest development version (as listed in the current `README.md`, does not solve the issue, as it still looks for `python`, which still does not exist.

One alternative that could resolve the issue would be to update `pidcat` to explicitly try to use `python3`, but this would have some less desirable backwards compatibility issues, perhaps.

Start with the interpreter line in pidcat.py and compare it with the current development version referenced by README.md. Reproduce the Homebrew-installed pidcat failure on macOS 12.3 and review the comment thread for compatibility constraints. Done means pidcat no longer looks for the missing interpreter while its supported installation behavior remains clear.

Written by the indexing model from the issue text.