matplotlib: port sed call to the main patch

This way we notice if something changes upstream.

mingw-w64-python-matplotlib/setupext.py.patch

@@ -1,6 +1,6 @@
---- matplotlib-3.1.0/setupext.py.orig	2019-05-18 03:55:28.000000000 +0200
-+++ matplotlib-3.1.0/setupext.py	2019-05-19 11:31:01.633173900 +0200
-@@ -24,6 +24,7 @@
+--- matplotlib-3.3.4/setupext.py.orig	2021-01-28 09:30:28.000000000 +0100
++++ matplotlib-3.3.4/setupext.py	2021-02-07 12:11:03.590667300 +0100
+@@ -20,6 +20,7 @@
  
  _log = logging.getLogger(__name__)
  
@@ -8,7 +8,19 @@
  
  def _get_xdg_cache_dir():
      """
-@@ -232,7 +233,7 @@
+@@ -146,9 +147,9 @@
+ options = {
+     'backend': config.get('rc_options', 'backend', fallback=None),
+     'system_freetype': config.getboolean(
+-        'libs', 'system_freetype', fallback=sys.platform.startswith('aix')),
++        'libs', 'system_freetype', fallback=sys.platform.startswith('aix') or MSYS),
+     'system_qhull': config.getboolean('libs', 'system_qhull',
+-                                      fallback=False),
++                                      fallback=False or MSYS),
+ }
+ 
+ 
+@@ -171,7 +172,7 @@
      """
      Get path to pkg-config and set up the PKG_CONFIG environment variable.
      """
@@ -17,7 +29,7 @@
          return None
      pkg_config = os.environ.get('PKG_CONFIG', 'pkg-config')
      if shutil.which(pkg_config) is None:
-@@ -276,7 +277,7 @@
+@@ -223,7 +224,7 @@
  
      # conda Windows header and library paths.
      # https://github.com/conda/conda/issues/2312 re: getting the env dir.