add new DPoPTokenSerializer class for [de]serializing DPoPToken

[snarfed]

for #122

@guillp one note here: this duplicates the existing code in AuthorizationRequestSerializer for [de]serializing DPoPKey. would you want a new DPoPKeySerializer class for that? I wasn't sure if you'd care more about the code duplication or the serializer class sprawl. I'm ok either way.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.94%. Comparing base (7fa1c7e) to head (0ca1b1e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
requests_oauth2client/dpop.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #174      +/-   ##
===========================================
- Coverage   100.00%   99.94%   -0.06%     
===========================================
  Files           19       19              
  Lines         1934     1955      +21     
===========================================
+ Hits          1934     1954      +20     
- Misses           0        1       +1     

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[snarfed]

@guillp friendly ping? I'd love to get back to using requests_oauth2client from upstream here instead of my fork!...but I need this PR first.