Bump Conan version to 1.28.0

[uilianries]

Changelog:

• Feature: Support Conan 1.28.0

• Fix: Env var CPT_TEST_FOLDER should be able to disable test_package (#518)

• [ ] Refer to the issue that supports this Pull Request.

• [ ] If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.

• [ ] I've read the Contributing guide.

• [ ] I've followed the PEP8 style guides for Python code.

[codecov-commenter]

Codecov Report

Merging #516 into master will decrease coverage by 0.01%. The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
- Coverage   92.84%   92.83%   -0.02%     
==========================================
  Files          35       35              
  Lines        3619     3641      +22     
==========================================
+ Hits         3360     3380      +20     
- Misses        259      261       +2     

Impacted Files	Coverage Δ
cpt/test/integration/basic_test.py	97.04% <95.00%> (-0.23%)	:arrow_down:
cpt/__init__.py	100.00% <100.00%> (ø)
cpt/runner.py	91.30% <100.00%> (+0.07%)	:arrow_up:
cpt/ci_manager.py	92.46% <0.00%> (-0.42%)	:arrow_down:

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