Update year range in copyright headers

[rahulporuri]

It's 2021 so the year range should be X-2021, not X-2020. The changes were done using automated regex, similar to what was done in the PR enthought/envisage#392. Regex search term - Copyright (\d{4})-2020 - replaced with Copyright $1-2021. Done on VS Code.

[codecov-io]

Codecov Report

Merging #854 (ecc2349) into master (4a991af) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #854      +/-   ##
==========================================
+ Coverage   41.19%   41.29%   +0.09%     
==========================================
  Files         522      522              
  Lines       28128    28128              
  Branches     4253     4253              
==========================================
+ Hits        11588    11616      +28     
+ Misses      16034    16015      -19     
+ Partials      506      497       -9     

Impacted Files	Coverage Δ
pyface/__init__.py	88.88% <ø> (+7.40%)	⬆️
pyface/about_dialog.py	100.00% <ø> (ø)
pyface/action/action.py	90.00% <ø> (ø)
pyface/action/action_controller.py	100.00% <ø> (ø)
pyface/action/action_event.py	100.00% <ø> (ø)
pyface/action/action_item.py	92.85% <ø> (ø)
pyface/action/action_manager.py	83.92% <ø> (ø)
pyface/action/action_manager_item.py	100.00% <ø> (ø)
pyface/action/api.py	100.00% <ø> (ø)
pyface/action/field_action.py	95.00% <ø> (ø)
... and 205 more

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[aaronayres35]

LGTM