Update ignore list and switch from Pyrex ('deprecated') to Cython.

.gitignore

@@ -9,3 +9,5 @@
 *.synctex.gz
 /build
 core
+.coverage
+coverage

external/pybison

@@ -1 +1 @@
-Subproject commit 0bdb26228364e4f7dda5e3537785727c02040b37
+Subproject commit 3e04c23b6a4d32db29a3d0c039abf33f1c8b3ce9

external/rules.mk

@@ -16,8 +16,6 @@ $(b)pybison/bisondynlib-linux.o $(b)pybison/bison_.o: | $(b)pybison
 $(b)pybison/bisondynlib-linux.o: $(d)pybison/src/c/bisondynlib-linux.c
 $(b)pybison/bison_.o: $(b)pybison/bison_.c
 
-PYREX := 1
-
 ifdef PYREX
 py2c := pyrexc
 else