fix(tracing): Use hasattr for _ctx_token

[szokeasaurusrex]

We currently use an if self._ctx_token check to determine if a span has a _ctx_token. However, this check is incorrect, since _ctx_token is only sometimes set. Furthermore, when _ctx_token is set, we should always have a _ctx_token.

Therefore, using hasattr is what we want.

---

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Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.87%. Comparing base (b07438d) to head (02cef85).
Report is 2 commits behind head on potel-base.

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Files with missing lines	Patch %	Lines
sentry_sdk/tracing.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           potel-base    #4541      +/-   ##
==============================================
- Coverage       84.87%   84.87%   -0.01%     
==============================================
  Files             144      144              
  Lines           15005    15005              
  Branches         2374     2374              
==============================================
- Hits            12736    12735       -1     
  Misses           1541     1541              
- Partials          728      729       +1     

Files with missing lines	Coverage Δ
sentry_sdk/tracing.py	80.41% <0.00%> (ø)

... and 1 file with indirect coverage changes