Add 'python3-devel' to the DNF bootstrap packages list

Bootstrapping on Fedora 31 fails with a missing <Python.h> header.
That header is provided by the 'python3-devel' package.

README.md

@@ -101,7 +101,7 @@ sudo dnf install curl libtool gcc-c++ libXi-devel libunwind-devel \
     libXrandr-devel gperf fontconfig-devel cabextract ttmkfdir  expat-devel \
     rpm-build openssl-devel cmake bzip2-devel libX11-devel libXcursor-devel \
     libXmu-devel mesa-libOSMesa-devel dbus-devel ncurses-devel harfbuzz-devel \
-    ccache mesa-libGLU-devel clang clang-libs gstreamer1-devel \
+    ccache mesa-libGLU-devel clang clang-libs gstreamer1-devel python3-devel \
     gstreamer1-plugins-base-devel gstreamer1-plugins-bad-free-devel autoconf213
 ```
 

python/servo/bootstrap.py

@@ -126,7 +126,7 @@ def linux(context, force=False):
                 'libXcursor-devel', 'libXmu-devel', 'mesa-libOSMesa-devel',
                 'dbus-devel', 'ncurses-devel', 'harfbuzz-devel', 'ccache',
                 'mesa-libGLU-devel', 'clang', 'clang-libs', 'gstreamer1-devel',
-                'gstreamer1-plugins-base-devel',
+                'gstreamer1-plugins-base-devel', 'python3-devel',
                 'gstreamer1-plugins-bad-free-devel', 'autoconf213']
     if context.distro == "Ubuntu" and context.distro_version != "14.04":
         pkgs_apt += ['libgstreamer1.0-dev', 'libgstreamer-plugins-base1.0-dev',