all: start v1.29.0-devel

Change-Id: I543d164519c1f8a095f9d8975213b182b3067f20
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/474436
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
diff --git a/internal/version/version.go b/internal/version/version.go
index f6e0119..02159d1 100644
--- a/internal/version/version.go
+++ b/internal/version/version.go
@@ -53,7 +53,7 @@
 	Major      = 1
 	Minor      = 29
 	Patch      = 0
-	PreRelease = ""
+	PreRelease = "devel"
 )
 
 // String formats the version string for this module in semver format.