Update Pywikipedia to Pywikibot and fix (C) lines

- Replace pywikibot with Pywikibot where possible.
- Standardise content and layout of copyright info at the top of files.
- Update year of last substantive code change on touched files.
- Revise the intro prose of externals/README

Change-Id: I8822e3f978f5113df778d835030f372d3c98e68e
Merge "Update Pywikipedia to Pywikibot and fix (C) lines"