Marking constants as Final

[deedy5]

From mypy docs:
You can use the typing.Final qualifier to indicate that a name or attribute should not be reassigned, redefined, or overridden. This is often useful for module and class level constants as a way to prevent unintended modification. Mypy will prevent further assignments to final names in type-checked code:

Testing after mypy compiling:

python3 setup.py --use-mypyc build_ext --inplace

Before commit:

------------------------------
--> Chardet Conclusions
   --> Avg: 0.1224901914893617s
   --> 99th: 0.73647s
   --> 95th: 0.17915s
   --> 50th: 0.01755s
------------------------------
--> Charset-Normalizer Conclusions
   --> Avg: 0.010322106382978723s
   --> 99th: 0.11215s
   --> 95th: 0.05355s
   --> 50th: 0.00428s
------------------------------
--> Charset-Normalizer / Chardet: Performance Сomparison
   --> Avg: x11.87
   --> 99th: x6.57
   --> 95th: x3.35
   --> 50th: x4.1

After commit:

------------------------------
--> Chardet Conclusions
   --> Avg: 0.12217872340425531s
   --> 99th: 0.72175s
   --> 95th: 0.17481s
   --> 50th: 0.01731s
------------------------------
--> Charset-Normalizer Conclusions
   --> Avg: 0.01016231914893617s
   --> 99th: 0.1085s
   --> 95th: 0.05219s
   --> 50th: 0.00419s
------------------------------
--> Charset-Normalizer / Chardet: Performance Сomparison
   --> Avg: x12.02
   --> 99th: x6.65
   --> 95th: x3.35
   --> 50th: x4.13

[codecov-commenter]

Codecov Report

Merging #208 (a18ab48) into 3.0 (0262569) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              3.0     #208      +/-   ##
==========================================
+ Coverage   90.05%   90.08%   +0.03%     
==========================================
  Files          11       11              
  Lines        1166     1170       +4     
==========================================
+ Hits         1050     1054       +4     
  Misses        116      116              

Impacted Files	Coverage Δ
charset_normalizer/constant.py	100.00% <100.00%> (ø)

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[Ousret]

We are close to the margin error regarding the performance gap. Using Final does not strike me as useful now.
I am considering dropping Python 3.6 but certainly not 3.7 and adding a mandatory dependency in order to exploit "Final" is a hard no for me.

[Ousret]

Adding a mandatory dependency for a single typing annotation bugs me.

[Ousret]

The test passes only because it requires installing all dev reqs. In live mode, it would crash because typing_ext is not found.