Time to prepare Stackless 3.8.

1. Merge branch 'main' into 'main-slp' up to commit 2ec872b Done
2. Cherry pick commits a11414b and 032a566 from 3.7-slp and close issue Add support for PEP 567 context variables to tasklets #239 Done
3. Merge branch 'main' into 'main-slp'. Merge tags v3.8.0a1, v3.8.0a2, v3.8.0a3. Bump the Stackless version to 3.8 after merging v3.8.0a1. Done
4. Merge commit 8c77b8c. Then test an merge pull request Stackless issue #275: Support PEP-523 eval_frame-hook #276.
5. Merge tags v3.8.0a4 and v3.8.0b1 and stop at commit 23d7ce7. Done
6. Last chance to merge pull request implement switching using the Stackman submodule #230 Done
7. Start branch 3.8-slp Done
8. Merge branch '3.8' into '3.8-slp'. Merge tags up and including to v3.8.0b4 Done
9. Merge commit bdace21. Then test and merge pull request Stackless issue #275: Support PEP-523 eval_frame-hook #276. Done
10. Merge branch '3.8' into '3.8-slp'. Merge tags up and including to v3.8.1. Done
11. Merge branch '3.8' into '3.8-slp' up to commit 5cadd3f
12. Cherry pick commit c3f61b0 from 3.7-slp and close issue Upstream fix for bpo-39386 breaks Stackless test case test_pickle.TestAsyncGenPickling #251
13. Merge branch '3.8' into '3.8-slp'. Merge all remaining tags. Starting with 3.8.6 use Sphinx 2.3.1 to build the documentation.