E3-core

* Minor backward incompatible changes:
* the discrimiant ``is_virtual`` has been removed
* e3.anod.sandbox.SandBox now has a mandatory root_dir attribute
* AnodSpecRepositories spec_config should now subclass SpecConfig
* PlanContext now returns PlanActionEnv, a subclass of BaseEnv. Contrary
to the previous BaseEnv objects, returned PlanActionEnv always have
the following attributes set: "push_to_store", "default_build",
"module", "source_packages"

* Add type hinting and verify it with ``mypy``
* Minor backward incompatible changes:
* some function argument are now mandatory:
* e3.anod.context.AnodContext.add_anod_action ``env``
* e3.anod.context.AnodContext.add_spec ``env`` and ``primitive``
* some attribute have been replaced by properties to avoid being marked as Optional
* e3.anod.spec.Anod ``build_space``
* e3.os.process out and err attributes preserve CR characters
* Deprecate e3.decorator.memoize, use functools.lru_cache instead
* Prevent crash when a process launched by e3.os.process.Run does not emit utf-8
on stdout or stderr. Output and error is now processed using bytes_as_str and
bytes version of output and error is available through raw_out and raw_err
attributes.

Convert code to support Python >= 3.7 only.

This is the last version supporting Python 2. Next major version will be Python 3 only.

Initial version