Use: https://github.com/JelleZijlstra/autotyping
to add "safe" return annotations.

Where a parameter has a default value that is an obvious scalar type
(bool, int, str, etc.) add those annotations as well.

Also fixed two small bugs that popped up when sanity-checking with
mypy.  One in FortranCommon, where a return had been previously
annotated to be a tuple of Action, which should be ActionBase -
Action is the factory function, not the base class. The other was
a typo in the error raised in _add_cppdefines - the message was
formatted with the value of "define" which should have been "defines".

Signed-off-by: Mats Wichmann <mats@linux.com>

Some nearby things in Main.py as well:
- docstrings polished a bit, minor linting
- move the list of predefined SConstruct file names into a constant
  defined at the top of the file, so it's a little less hidden,
  in the unlikely case of future changes.

Manpage text and example revised a bit.

Signed-off-by: Mats Wichmann <mats@linux.com>

The new ValidateOptions had a :versionadded: in the docstring,
but just noticed the actual manpage entry didn't have similar.

Signed-off-by: Mats Wichmann <mats@linux.com>

* Reworded the section on --experimental - separate when the two current
  features were made available, and when --experimental itself was added.
* Reworded in GetOption
* Changed tags on a few options
* Use entity forms in more places (Python -> &Python;, SConscript ->
  &SConscript; etc.)
* Make changed/added notifications more consistent with what Sphinx
  produces for the API docs (:versionadded: and :versionchanged: markup)

Signed-off-by: Mats Wichmann <mats@linux.com>