Tweak Util module

[mwichmann]

Move UtilTests.py from the top level to the Util package directory, for consistency with the other packages with unittests.

Renamed Util/types.py -> Util/sctypes.py. types is the name of a stdlib module and it's a bad idea to duplicate it, even though in this case it was "legal" since our file was not at the top level. (Moving UtilTests.py actually made this a real problem)

Class Selector is no longer an OrderedDict, it just inherits from dict as ordering is now preserved and we never used any extra features of OrderedDict.

Fix API doc build - was missing a good bit of Util since it was split into a package.

Moved the import-loop warning to the top of __init__.py so it will be more visible.

Fiddly linting, doc-stringing, etc. Super-fiddly: pylint flags foo, bar and baz as prohibited variable/function/method names. Actually changed these in UtilTests.py.

SCons.Errors has been a source of import loops because it imports SCons.Util. Now Util is split, directly import from the SCons.Util.sctypes submodule the two things Errors needs - this may reduce the chance of import problems.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation

[bdbaddog]

One concern here is that out in the wild folks are doing
from SCons.Util.types import is_string or similar instead of doing so from SCons.Util.

So minimally this change should explicitly explained in CHANGES and RELEASE.

I'll do some searching of our more notable users git repos and see what's the situation..

[mwichmann]

SCons.Util only became a package in 4.5, before that, there was no SCons.Util.types to import from. Trying to fix my poor choice at the time.

Edit: it seems unlikely that anyone would have picked up importing this in the last six months: it was created such that the symbols continued to be made available just by importing SCons.Util, so there would not have been any "not found" event to trigger someone to go looking. Yes, by looking at the source after seeing the release note you can see that there are now distinct module files in the new package, but I'd be surprised if anyone started using for that reasons.