[roll third_party/pigweed] pw_env_setup: Upgrade sphinx and dependencies for docs

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/168431

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 21c317d5c139c3f..b7a3673c074997f
Roller-URL: https://ci.chromium.org/b/8771840472418721377
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie3a42386fa771de8f175379844268ac1f989ff4e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/168552
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 21c317d..b7a3673 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 21c317d5c139c3f72e701d87ea3a8139428531a3
+Subproject commit b7a3673c074997f0011732393d10868213effa48