Replace `self._canvas` and `self.scanvas`, both bound to `canvas`,
with `self.canvas, which is accessed in other methods.
Replace `_s_` with `screen` and `_s_._canvas` with `canvas`.
Add a comment explaining the unorthodox use of
function turtle.Screen and singleton class turtle._Screen.

Fixed flickering when the tracer is turned off.

On macOS, tk defers to system setting for button background when in normal state.
Give turtledemo button text a color that works on either light or dark background.

***   File "/usr/lib64/python3.9/turtledemo/__main__.py", line 275
        bg="#d00" if clear == NORMAL else"#fca")
                                        ^
    SyntaxError: invalid string prefix

This is the converse of GH-15353 -- in addition to plenty of
scripts in the tree that are marked with the executable bit
(and so can be directly executed), there are a few that have
a leading `#!` which could let them be executed, but it doesn't
do anything because they don't have the executable bit set.

Here's a command which finds such files and marks them.  The
first line finds files in the tree with a `#!` line *anywhere*;
the next-to-last step checks that the *first* line is actually of
that form.  In between we filter out files that already have the
bit set, and some files that are meant as fragments to be
consumed by one or another kind of preprocessor.

    $ git grep -l '^#!' \
      | grep -vxFf <( \
          git ls-files --stage \
          | perl -lane 'print $F[3] if (!/^100644/)' \
        ) \
      | grep -ve '\.in$' -e '^Doc/includes/' \
      | while read f; do
          head -c2 "$f" | grep -qxF '#!' \
          && chmod a+x "$f"; \
        done

There are plenty of legitimate scripts in the tree that begin with a
`#!`, but also a few that seem to be marked executable by mistake.

Found them with this command -- it gets executable files known to Git,
filters to the ones that don't start with a `#!`, and then unmarks
them as executable:

    $ git ls-files --stage \
      | perl -lane 'print $F[3] if (!/^100644/)' \
      | while read f; do
          head -c2 "$f" | grep -qxF '#!' \
          || chmod a-x "$f"; \
        done

Looking at the list by hand confirms that we didn't sweep up any
files that should have the executable bit after all.  In particular

 * The `.psd` files are images from Photoshop.

 * The `.bat` files sure look like things that can be run.
   But we have lots of other `.bat` files, and they don't have
   this bit set, so it must not be needed for them.



Automerge-Triggered-By: @benjaminp

Actually time calculations were remove in commit 891a1f8.

The 'wikipedia' example is now 'rosette', describing what it draws.
The 'penrose' print output is reduced.  The 'tree' '1024'
output is eliminated.