Regenerate CI workflows with xmsconan 2.15.3

[wdolinar]

Regenerates CI and build files with xmsconan 2.15.3 (was 2.15.2).

• XmsCore-CI.yaml: flake job now installs xmsconan, runs xmsconan_gen build.toml to render .flake8, and runs plain flake8 _package — the inline flake8 arguments are gone, so .flake8 is the single lint source of truth.
• Build/deploy steps pin xmsconan==2.15.3 (was >=) and conan~=2.31.0 (was unpinned).
• Coverage.yaml intentionally keeps xmsconan>=.
• .flake8: adds pydocs/source/conf.py to exclude, drops isolated = true.

CMakeLists.txt is unchanged — xmscore was already on 2.15.2, which carried the source_group/IS_EMSCRIPTEN_BUILD changes.

Release notes: https://github.com/Aquaveo/xmsconan/releases/tag/2.15.3