Factor out code for configuring text widget colors to a new function.

Patch by Anish Shah and Aatish Neupane.

This matches the usage of ZipFile and BufferedWriter. This still requires
return values to be bytes() objects.

Also document and test that the write() methods should only access their
argument before they return.

Based on documentation by AM Kuchling.

Check the user warning in test_error_after_default() to not pollute the output,
and check the warning logged into stderr.

Based on patch by Alex Shkop.

Most fixes to Doc/ and Lib/ directories by Ville Skyttä.

Patch by Xavier de Gaye.

Such fonts on Linux prevented the configuration dialog from opening.

Patch by Ville Skyttä.

Patch by A. Jesse Jiryu Davis.

format unit.

Issue #26741: asyncio: BaseSubprocessTransport._process_exited() now copies the
return code from the child watched to the returncode attribute of the Popen
object. On Python 3.6, it is required to avoid a ResourceWarning.

These added a path attribute to pathlib.Path objects, and docs.
Instead, we're going to use PEP 519.

(Starting in the 3.4 branch and merging forward from there since that's what I did originally.)