cffi: fix build on Python 3

CFFI expects the arguments to be of type str, which means that the string
literals should not have the `b` prefix.

mercurial/cffi/bdiffbuild.py

@@ -5,10 +5,10 b' import os'
 
 ffi = cffi.FFI()
 with open(
-    os.path.join(os.path.join(os.path.dirname(__file__), b'..'), b'bdiff.c')
+    os.path.join(os.path.join(os.path.dirname(__file__), '..'), 'bdiff.c')
 ) as f:
     ffi.set_source(
-        b"mercurial.cffi._bdiff", f.read(), include_dirs=[b'mercurial']
+        "mercurial.cffi._bdiff", f.read(), include_dirs=['mercurial']
     )
 ffi.cdef(
     """

mercurial/cffi/mpatchbuild.py

@@ -5,11 +5,11 b' import os'
 
 ffi = cffi.FFI()
 mpatch_c = os.path.join(
-    os.path.join(os.path.dirname(__file__), b'..', b'mpatch.c')
+    os.path.join(os.path.dirname(__file__), '..', 'mpatch.c')
 )
 with open(mpatch_c) as f:
     ffi.set_source(
-        b"mercurial.cffi._mpatch", f.read(), include_dirs=[b"mercurial"]
+        "mercurial.cffi._mpatch", f.read(), include_dirs=["mercurial"]
     )
 ffi.cdef(
     """

mercurial/cffi/osutilbuild.py

@@ -4,7 +4,7 b' import cffi'
 
 ffi = cffi.FFI()
 ffi.set_source(
-    b"mercurial.cffi._osutil",
+    "mercurial.cffi._osutil",
     """
 #include <sys/attr.h>
 #include <sys/vnode.h>
@@ -22,7 +22,7 b' typedef struct val_attrs {'
     off_t             datalength;
 } __attribute__((aligned(4), packed)) val_attrs_t;
 """,
-    include_dirs=[b'mercurial'],
+    include_dirs=['mercurial'],
 )
 ffi.cdef(
     '''