[roll pigweed] pw_protobuf_compiler: Make the yapf import optional

yapf is a significant requirement and is not essential for proto_repr()
to operate. Gracefully handle missing yapf imports and remove yapf from
install_requires for pw_protobuf_compiler.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100661

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 4be62b18a933a50..c50e44f1590eccb
Roller-URL: https://ci.chromium.org/b/8809821497312287953
Cq-Cl-Tag: roller-builder:pigweed-zephyr-roller
Cq-Cl-Tag: roller-bid:8809821497312287953
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id1513a927a8d002b217b38adf42abd5138634ad6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/100742
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pigweed b/pigweed
index 4be62b1..c50e44f 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 4be62b18a933a5084ce8e2cfce5cc128067584cf
+Subproject commit c50e44f1590eccbbaaf71243aa5e9c1a6532a465