API Changes - Simplify EnvironmentMixin

[tony]

Via #389

• Removed key lookups from {meth}libtmux.common.EnvironmentMixin.show_environment

  Only EnvironmentMixin.show_environment() (without an argument) exists, and
  it still returns a dict.

• Add key lookups via {meth}libtmux.common.EnvironmentMixin.getenv

  # Before
  server.show_environment('DISPLAY')
  
  # After
  server.getenv('DISPLAY')
  
  # Before
  session.show_environment('DISPLAY')
  
  # After
  session.getenv('DISPLAY')

• Removed key lookups from {meth}Session.show_options

  session.show_options()  # still returns dict, without an argument
  
  # Old
  session.show_options('DISPLAY')
  
  # Now
  session.show_option('DISPLAY')

• Removed key lookups from {meth}Window.show_window_options

  window.show_window_options()  # still returns dict, without an argument
  
  # Old
  window.show_window_options('DISPLAY')
  
  # Now
  window.show_window_option('DISPLAY')

[codecov]

Codecov Report

Merging #390 (fc82e28) into master (f606a42) will increase coverage by 0.00%.
The diff coverage is 97.56%.

❗ Current head fc82e28 differs from pull request most recent head 2a3bea2. Consider uploading reports for the commit 2a3bea2 to get more accurate results

@@           Coverage Diff           @@
##           master     #390   +/-   ##
=======================================
  Coverage   88.08%   88.08%           
=======================================
  Files          16       16           
  Lines        1611     1620    +9     
=======================================
+ Hits         1419     1427    +8     
- Misses        192      193    +1     

Impacted Files	Coverage Δ
libtmux/common.py	86.97% <94.11%> (+0.21%)	⬆️
libtmux/session.py	80.70% <100.00%> (-0.23%)	⬇️
libtmux/window.py	84.11% <100.00%> (-0.19%)	⬇️
tests/test_server.py	100.00% <100.00%> (ø)
tests/test_session.py	98.55% <100.00%> (ø)
tests/test_window.py	98.78% <100.00%> (ø)

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