Simplify installation and upgrades with pipx

[cclauss]

From the Python Packaging Authority: https://pypa.github.io/pipx and https://github.com/pypa/pipx

pipx is already in use in this repo: https://github.com/dosisod/refurb/blob/master/.github/workflows/deploy.yml#L21

[dosisod]

Hi @cclauss , thank you for bring this up! I have always just used pip, but I know pipx and poetry are some of the more popular alternatives. I think that if we are going to include pipx, we might as well include poetry. Also, to make it clear which package manager we are talking about, could we change it to look something more like this?

## Installing

Refurb can be installed many different ways blah blah blah:

### Pipx

<Insert instructions here>

### Poetry

<Insert instructions here>

### Pip

<Insert instructions here>

Also, since the refurb file.py folder/ line is the same for each section, perhaps we could add a Running Refurb section after the installation section.

I'm open to suggestions!

[dosisod]

Closed via #277. Sorry for the delay on this!