RBTools sometimes needs to invoke editors (to create commit messages,

for instance). The code we had before failed on Python 3, due to string

type mismatches when reading in the resulting edited file content, but

even with that fixed, the editing experience was error-prone and far

from ideal.
For instance, if things went wrong during edit_text(), we'd get some

generic exception, like OSError. We now have a dedicated error for

editing-related failures, EditorError.
Another limitation was that we had no control over the filename of the

edited file. This meant we couldn't influence the editor -- no syntax

highlighting or other editor settings would apply, creating a subpar

experience. This is currently most important for editing commit

messages.
To address this, we needed to provide a filename during the editing

process, but it wasn't as simple as just a standard filename. We needed

to support two different ways of doing this: The Git way (using a fixed

filename for the file), and the Mercurial way (using a generated temp

filename with a prefix and a suffix).
edit_text() has been updated to make the Git approach easier. It now

takes a filename, which will be passed to make_tempfile() to generate

a specific filename for the content. It will also immediately delete

that file once it's been read back in, just to keep the filesystem a bit

more tidy.
To support the Mercurial approach, a new method was added:

edit_file(). This contains the core logic of edit_text(), but

operates on an existing filename. This means that for more advanced

cases, like with Mercurial commit messages, we can generate a commit

filename elsewhere and then pass it to edit_file().
These new editing capabilities have one additional benefit: The

configured editor can now be specified as RBTOOLS_EDITOR, letting us

define an editor that isn't dependent on the VISUAL or EDITOR

environment variables. This is mostly useful for testing for now, but

can be useful if someone wants an explicit editor to be used for RBTools

(or a wrapper script that processes text in some way).
A dummy editor has been added inside rbtools/testing/scripts/ for

testing purposes, allowing us to have full test coverage of these

methods. It simply uppercases the whole file.

Unit tests pass.
Manually tested the new editor errors and capabilities alongside an

upcoming change to leverage all this in Git.