Update README's supported Ubuntu/Fedora versions

[rmartin16]

Changes

• Yet more updates to Linux requirements....specifically that Ubuntu 20.04+ and Fedora 32+ is required

Notes

• The requirements for Toga on Linux are more complex now than GTK>=3.22
• The primary driver of Toga's base requirements for Linux is the oldest supported version of PyGObject: 3.46.0
• PyGObject 3.46.0 requires:
  • glib >= 2.64.0
  • GObject Introspection >= 1.64.0
  • pycairo >=1.16.0
    • pycairo 1.17.0 (i.e. Toga's minimum version) requires cairo >= 1.13.1
    • but more recent pycairo 1.26.0 requires cairo >= 1.15.10
• While experimenting with different distro releases, I hit issues for each one of these being too old...
• Of these, though, the most likely issue will probably be glib2 being too old
• I would add these specific versions to the Toga docs....but it just seems they will become inevitably out of date while also being of little value because most people use modern distro releases

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

FYI: Pillow is no longer building on 3.13.0b1; waiting on:

• Parse _version contents instead of using exec() python-pillow/Pillow#8050

[freakboy3742]

Thanks - LGTM.

Regarding the Pillow failure - I guess we'll just have to live with that for the short term.