Remove support for PyPy

[jrbourbeau]

In the monthly community meeting on 2022-02-03, we decided to intentionally remove support for PyPy (pasted the corresponding section from the meeting notes below). This PR removes all PyPy support code.

Relevant monthly meeting notes:

• Drop PyPy? Drop PyPy compatibility? #5681
  • We don’t see any users here
  • There is some mild cost
  • We don’t really support this today
  • We’re now going to intentionally not support it.

• Closes Drop PyPy compatibility? #5681
• Tests added / passed
• Passes pre-commit run --all-files

cc @jakirkham @mrocklin for visibility

[mrocklin]

+1

[github-actions]

Unit Test Results

       18 files  +       4         18 suites  +4   9h 10m 49s ⏱️ + 1h 10m 14s
  2 703 tests ±       0    2 621 ✔️ +       4       81 💤  -     2  1 ❌  - 2 
24 163 runs  +5 354  22 875 ✔️ +5 072  1 287 💤 +284  1 ❌  - 2 

For more details on these failures, see this check.

Results for commit 550588f. ± Comparison against base commit 8906cab.

[jcrist]

Test failed due to some flakiness in the GPU-CI infrastructure (all tests passed, then something else failed afterwards). Merging.