python,python3: simplify `sed` expression for setuptools & pip

A suggestion from comment (from Philip Prindeville):
https://github.com/openwrt/packages/issues/4529#issuecomment-312518121

Avoids some escaping of slashes.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python/files/python-package-pip.mk

@@ -15,7 +15,7 @@ endef
 define PyPackage/python-pip/install
 	$(INSTALL_DIR) $(1)/usr/bin $(1)/usr/lib/python$(PYTHON_VERSION)/site-packages
 	# Adjust shebang to proper python location on target
-	sed "1s/.*/#\!\/usr\/bin\/python$(PYTHON_VERSION)/" -i $(PKG_BUILD_DIR)/install-pip/bin/*
+	sed "1s@.*@#\!/usr/bin/python$(PYTHON_VERSION)@" -i $(PKG_BUILD_DIR)/install-pip/bin/*
 	$(CP) $(PKG_BUILD_DIR)/install-pip/bin/* $(1)/usr/bin
 	$(CP) \
 		$(PKG_BUILD_DIR)/install-pip/lib/python$(PYTHON_VERSION)/site-packages/pip \

lang/python/python/files/python-package-setuptools.mk

@@ -15,7 +15,7 @@ endef
 define PyPackage/python-setuptools/install
 	$(INSTALL_DIR) $(1)/usr/bin $(1)/usr/lib/python$(PYTHON_VERSION)/site-packages
 	# Adjust shebang to proper python location on target
-	sed "1s/.*/#\!\/usr\/bin\/python$(PYTHON_VERSION)/" -i $(PKG_BUILD_DIR)/install-setuptools/bin/*
+	sed "1s@.*@#\!/usr/bin/python$(PYTHON_VERSION)@" -i $(PKG_BUILD_DIR)/install-setuptools/bin/*
 	$(CP) $(PKG_BUILD_DIR)/install-setuptools/bin/* $(1)/usr/bin
 	$(CP) \
 		$(PKG_BUILD_DIR)/install-setuptools/lib/python$(PYTHON_VERSION)/site-packages/pkg_resources \

lang/python/python3/files/python3-package-pip.mk

@@ -15,7 +15,7 @@ endef
 define Package/python3-pip/install
 	$(INSTALL_DIR) $(1)/usr/bin $(1)/usr/lib/python$(PYTHON3_VERSION)/site-packages
 	# Adjust shebang to proper python location on target
-	sed "1s/.*/#\!\/usr\/bin\/python$(PYTHON3_VERSION)/" -i $(PKG_BUILD_DIR)/install-pip/bin/*
+	sed "1s@.*@#\!/usr/bin/python$(PYTHON3_VERSION)@" -i $(PKG_BUILD_DIR)/install-pip/bin/*
 	$(CP) $(PKG_BUILD_DIR)/install-pip/bin/pip3* $(1)/usr/bin
 	$(CP) \
 		$(PKG_BUILD_DIR)/install-pip/lib/python$(PYTHON3_VERSION)/site-packages/pip \

lang/python/python3/files/python3-package-setuptools.mk

@@ -15,7 +15,7 @@ endef
 define Py3Package/python3-setuptools/install
 	$(INSTALL_DIR) $(1)/usr/bin $(1)/usr/lib/python$(PYTHON3_VERSION)/site-packages
 	# Adjust shebang to proper python location on target
-	sed "1s/.*/#\!\/usr\/bin\/python$(PYTHON3_VERSION)/" -i $(PKG_BUILD_DIR)/install-setuptools/bin/*
+	sed "1s@.*@#\!/usr/bin/python$(PYTHON3_VERSION)@" -i $(PKG_BUILD_DIR)/install-setuptools/bin/*
 	$(CP) $(PKG_BUILD_DIR)/install-setuptools/bin/easy_install-* $(1)/usr/bin
 	$(LN) easy_install-$(PYTHON3_VERSION) $(1)/usr/bin/easy_install-3
 	$(CP) \