Issue 25802: Finish deprecating load_module()

There are a couple concrete implementations of load_module() in importlib that have not been explicitly deprecated that probably should be. As part of this I will finally add an Examples section to the importlib docs to show how to do common tasks such as load a source file.

New changeset ab0e60a36368 by Brett Cannon in branch 'default':
Issue #25802: Deprecate load_module() on importlib.machinery.SourceFileLoader and SourcelessFileLoader.
https://hg.python.org/cpython/rev/ab0e60a36368

I'm leaving this issue open until I add an Examples section to the importlib docs. I will use the imp module as a guide of what to document.

Here is my attempt at adding an Examples section to the importlib docs. I cover importlib.util.find_spec(), importlib.import_module(), how to import a module by file path, and give an approximate import implementation for illustrative purposes.

issue25802.diff looks good to me (just added a minor comment on Rietveld)

New changeset d733b6275e17 by Brett Cannon in branch 'default':
Issue #25802: Add an examples section to importlib.
https://hg.python.org/cpython/rev/d733b6275e17

Thanks for the review, Berker! I managed to figure out how to get the Examples links to work so that made it into the final version.

messages: + msg257781