v4.20.209: test: assert requester post runs on requester loop (#1185)

What's Changed

• fix: work around mypyc package getattr segfault by @BobTheBuidler in #1170
• fix: track legacy wait_for DebuggableFuture waiters by @BobTheBuidler in #1178
• fix: track shielded DebuggableFuture waiters by @BobTheBuidler in #1179
• fix: avoid no-session requester shutdown wait by @BobTheBuidler in #1184
• perf: mark acyclic mypyc classes for faster GC by @BobTheBuidler in #1168
• perf: use librt time in compiled paths by @BobTheBuidler in #1181
• docs: add segfault debugging skill by @BobTheBuidler in #1176
• chore: bump mypyc to v2.0.0 by @BobTheBuidler in #1180

Full Changelog: v4.20.208...v4.20.209