all: start v1.29.1-devel

Change-Id: Ie844955201027d59486a14e2470ffbac5bce5da8
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/475877
Reviewed-by: Michael Stapelberg <stapelberg@google.com>

internal/version/version.go

@@ -53,7 +53,7 @@ const (
 	Major      = 1
 	Minor      = 29
 	Patch      = 1
-	PreRelease = ""
+	PreRelease = "devel"
 )
 
 // String formats the version string for this module in semver format.