- Add missing Requires on two modules.
- Drop no longer required appdirs Requires.

- Switch off tests, they are just broken.

- Update to 20.7.0:
- Removed xonsh activator due to this breaking fairly often the
CI and lack of support from those packages maintainers,
upstream is encouraged to continue supporting the project as
a plugin
- Support Python interpreters without distutils (fallback to
syconfig in these cases)
- Plugins now use 'selectable' entry points
- add libffi-7.dll to the hard-coded list of dlls for PyPy
- Drop python 3.4 support as it has been over 2 years since EOL
- Use the better maintained platformdirs instead of appdirs
- Built in discovery class is always preferred over plugin
supplied classes.
- On the programmatic API allow passing in the environment
variable dictionary to use, defaults to os.environ if not
specified
- The builtin discovery takes now a --try-first-with argument
and is first attempted as valid interpreters. One can use
this to force discovery of a given python executable when the
discovery order/mechanism raises errors