python-overrides/lxml: fix build w/ libxml-2.15

Closes #76

components/fix-lxml-libxml-2.15-build.patch

@@ -0,0 +1,12 @@
+diff --git a/src/lxml/etree.c b/src/lxml/etree.c
+index 9fbfe5a..74c83c7 100644
+--- a/src/lxml/etree.c
++++ b/src/lxml/etree.c
+@@ -35183,7 +35183,6 @@ static void __pyx_f_4lxml_5etree_fixThreadDictNamesForDtd(xmlDtd *__pyx_v_c_dtd,
+  *                 _fixThreadDictPtr(&c_attribute.name, c_src_dict, c_dict)
+  *                 _fixThreadDictPtr(&c_attribute.prefix, c_src_dict, c_dict)
+ */
+-        __pyx_f_4lxml_5etree__fixThreadDictPtr((&__pyx_v_c_attribute->defaultValue), __pyx_v_c_src_dict, __pyx_v_c_dict);
+ 
+         /* "src/lxml/proxy.pxi":578
+  *             while c_attribute:

components/python-overrides.nix

@@ -77,6 +77,7 @@ let
     lxml = prev.lxml.overrideAttrs (
       {
         buildInputs ? [ ],
+        patches ? [ ],
         ...
       }:
       {
@@ -85,6 +86,13 @@ let
           libxml2
           zlib
         ];
+        patches = patches ++ [
+          # The upstream fix for this is
+          # https://github.com/lxml/lxml/commit/f7a813483c4482dd114e7ee8b42b54337e285503,
+          # however, this doesn't help us here because the `etree.c` file is already generated
+          # (we're using the wheel), so we have to patch the C file directly.
+          ./fix-lxml-libxml-2.15-build.patch
+        ];
       }
     );
     xmlsec = prev.xmlsec.overrideAttrs (