Use symbiflow's symbolator

Signed-off-by: Daniel Lim Wee Soong <weesoong.lim@gmail.com>

docs/requirements.txt

diff --git a/docs/requirements.txt b/docs/requirements.txt
index ae4e343..7d20d0d 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -14,7 +14,7 @@
 sphinxcontrib-verilog-diagrams
 
 # Module diagrams
-symbolator
+git+https://github.com/SymbiFlow/symbolator.git#egg=symbolator
 
 pycairo
 # vext.gi