fix #744 #843 and disable E203 (flake8).

fix #744 #843 and disable E203 (flake8).

Indeed, before this patch, no `# Title: XYZ` was describing our generated
host file. This patch fixes that.

Indeed, we now do the following:

- If no extension is included we add:
    - `# Title: StevenBlack/hosts`
- If exactly one extension is included we add:
    - `# Title: StevenBlack/hosts with the extension_name extension`
- If more than one extension is included we add (in one line):
    - `# Title: StevenBlack/hosts with the extname1,
      extname2 and extname3 extensions`

Please also note that I disable E203 (whitespace before ':') for flake8
at line 773.

Since we use Black for code formatting let me redirect you to
the documentation of Black which explain what I did.

https://black.readthedocs.io/en/stable/the_black_code_style.html#slices :

> Since E203 is not PEP 8 compliant, you should tell
> Flake8 to ignore these warnings.