Enforce private instantiation and inheritance through metaclass

[ankitkumarr]

Issue: #1021

Added two new metaclasses:

1. Final: Ensures that no class can inherit the class using the metaclass
2. NoPublicConstructor: Ensures that no object of the class using the metaclass can be directly created. This metaclass derives from Final, also ensuring 1

[oremanj]

Thanks, this generally looks good! Just a couple of comments

[codecov]

Codecov Report

Merging #1043 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1043      +/-   ##
==========================================
- Coverage   99.53%   99.45%   -0.09%     
==========================================
  Files         102      102              
  Lines       12287    12317      +30     
  Branches      915      916       +1     
==========================================
+ Hits        12230    12250      +20     
- Misses         36       43       +7     
- Partials       21       24       +3

Impacted Files	Coverage Δ
trio/tests/test_util.py	100% <100%> (ø)	⬆️
trio/_core/_exceptions.py	100% <100%> (ø)	⬆️
trio/_core/tests/test_run.py	99.86% <100%> (-0.14%)	⬇️
trio/_util.py	95.28% <100%> (-4.72%)	⬇️
trio/_core/_run.py	99.7% <100%> (ø)	⬆️
trio/_core/tests/test_ki.py	99.02% <0%> (-0.98%)	⬇️