python*: remove dependency to [sed]

Even if 'sed -i' is not posix and we should not use system sed this way:
- even busybox supports this option
- we already use it in so many other places that it does not make sense to
  to impose ourselves such extra dependency

If we plan to fix all 'sed -i', alternatives to [sed] are:
- patch
- 'perl -pi -e' (perl is already built for openssl)

And we should first have a mechanism to prevent misuse of system sed.

component/python-2.7/buildout.cfg

@@ -9,7 +9,6 @@ extends =
   ../openssl/buildout.cfg
   ../patch/buildout.cfg
   ../readline/buildout.cfg
-  ../sed/buildout.cfg
   ../sqlite3/buildout.cfg
   ../zlib/buildout.cfg
   ../file/buildout.cfg
@@ -51,7 +50,7 @@ patches = ${python2.7-lib-patches:patches}
 url =
   http://www.python.org/ftp/python/${:package_version}/Python-${:package_version}${:package_version_suffix}.tar.xz
 pre-configure =
-  ${sed:location}/bin/sed -i -e "s/if 'curses' in ln:/if 'curses' in ln.split(os.sep)[-1]:/" setup.py
+  sed -i -e "s/if 'curses' in ln:/if 'curses' in ln.split()[0]:/" setup.py
 configure-options =
   --enable-ipv6
   --enable-unicode=ucs4

component/python3/buildout.cfg

@@ -10,7 +10,6 @@ extends =
   ../ncurses/buildout.cfg
   ../openssl/buildout.cfg
   ../readline/buildout.cfg
-  ../sed/buildout.cfg
   ../sqlite3/buildout.cfg
   ../util-linux/buildout.cfg
   ../xz-utils/buildout.cfg
@@ -32,7 +31,7 @@ executable = @@LOCATION@@/bin/${:_buildout_section_name_}
 url =
   https://www.python.org/ftp/python/${:package_version}/Python-${:package_version}${:package_version_suffix}.tar.xz
 pre-configure =
-  ${sed:location}/bin/sed -i -e "s/if 'curses' in ln:/if 'curses' in ln.split(os.sep)[-1]:/" setup.py
+  sed -i -e "s/if 'curses' in ln:/if 'curses' in ln.split()[0]:/" setup.py
 configure-options =
   --enable-ipv6
   --without-ensurepip