tty: restore cursor when stopped

[jkloetzke]

If the user presses Ctrl+Z on the terminal, the kernel will send SIGTSTP to the current foreground process group. By default, this will directly stop all processes. But if we have messed with the terminal (e.g. disabled cursor), this will leave the terminal in a barely usable state.

Fix that by handling SIGTSTP and restore the original tty settings. If that is done, we can stop ourself. Upon resumption, re-initialize the tty and carry on.

[codecov]

Codecov Report

❌ Patch coverage is 44.82759% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.25%. Comparing base (97b3aa2) to head (3957e86).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pym/bob/tty.py	44.82%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
- Coverage   89.28%   89.25%   -0.03%     
==========================================
  Files          50       50              
  Lines       16422    16443      +21     
==========================================
+ Hits        14662    14676      +14     
- Misses       1760     1767       +7     

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