Build-Depends: replace squeeze specific python-sip4-dev with generic python-sip-dev

python-sip4-dev is available in squeeze only and fails to build
on wheezy:

| % rmadison python-sip4-dev
|  python-sip4-dev | 4.10.2-1 | squeeze | all

So use the python-sip-dev package instead:

| % rmadison python-sip-dev
|  python-sip-dev | 4.10.2-1 | squeeze      | amd64, armel, i386, ia64, kfreebsd-amd64, kfreebsd-i386, mips, mipsel, powerpc, s390, sparc
|  python-sip-dev | 4.13.3-2 | wheezy       | amd64, armel, armhf, i386, ia64, kfreebsd-amd64, kfreebsd-i386, mips, mipsel, powerpc, s390, s390x, sparc
|  python-sip-dev | 4.13.3-2 | sid          | amd64, armel, armhf, hurd-i386, i386, ia64, kfreebsd-amd64, kfreebsd-i386, mips, mipsel, powerpc, s390, s390x, sparc
|  python-sip-dev | 4.14.3-1 | experimental | amd64, armel, armhf, hurd-i386, i386, ia64, kfreebsd-amd64, kfreebsd-i386, mips, mipsel, powerpc, s390, s390x, sparc

debian/control

@@ -14,7 +14,7 @@ Build-Depends: debhelper (>= 7),
                libxml2-dev,
                openssl,
                python-dev,
-               python-sip4-dev,
+               python-sip-dev,
                sip4
 Standards-Version: 3.9.3