Deprecate safeweakref and replace its uses

[mdickinson]

Some safeweakref-related cleanup, aimed at eventual removal of the safeweakref module.

• Don't try to make safeweakref a drop-in replacement for weakref: safeweakref no longer does from weakref import *
• Deprecate the existing safeweakref.ref
• Replace existing uses of safeweakref with a simpler, private version that still handles bound methods specially, but doesn't do the caching that safeweakref does
• Add tests for the deprecation of safeweakref.ref
• Add tests to cover the uses of that simpler replacement.
• Make safeweakref flake8-clean, and remove it from the flake8 exclusion list.

Closes #271

[codecov-io]

Codecov Report

Merging #275 into master will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master    #275      +/-   ##
=========================================
+ Coverage   53.78%   53.8%   +0.01%     
=========================================
  Files         135     135              
  Lines        4397    4394       -3     
  Branches      511     511              
=========================================
- Hits         2365    2364       -1     
+ Misses       1991    1989       -2     
  Partials       41      41

Impacted Files	Coverage Δ
...visage/plugins/python_shell/view/namespace_view.py	0% <ø> (ø)	⬆️
envisage/ui/workbench/workbench_application.py	58.69% <ø> (-0.88%)	⬇️
envisage/ui/tasks/action/exit_action.py	0% <ø> (ø)	⬆️
envisage/plugins/event_manager/plugin.py	0% <0%> (ø)	⬆️
envisage/ui/tasks/preferences_pane.py	53.57% <100%> (ø)	⬆️

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[mdickinson]

@corranwebster I've been self-merging a good few Envisage PRs (bad Mark!), but this one could probably use a review, if you have the bandwidth.

[corranwebster]

LGTM