Python: remove pythonSmall

In #19309 a separate output for tkinter was added.

Several dependencies of Python depend indirectly on Python. We have the
following two paths:
```
‘python-2.7.12’ - ‘tk-8.6.6’ - ‘libXft-2.3.2’ - ‘libXrender-0.9.10’ -
‘libX11-1.6.4’ - ‘libxcb-1.12’ - ‘libxslt-1.1.29’- ‘libxml2-2.9.4’ -
‘python-2.7.12’

‘python-2.7.12’ - ‘tk-8.6.6’ - ‘libXft-2.3.2’ - ‘fontconfig-2.12.1’ -
‘dejavu-fonts-2.37’ - ‘fontforge-20160404’ - ‘python-2.7.12’
```
Because only `tkinter` needs this, I added
```
pythonSmall = python.override {x11Support = false;};
```
to break the infinite recursion. We also still have the output
`tkinter`.

However, we might as well build without x11Support by default. Then we build with x11Support as well so we get the tkinter module and put that in a separate package.

doc/languages-frameworks/python.md

@@ -422,7 +422,7 @@ exists.
 
 #### Missing `tkinter` module standard library
 
-To reduce closure size the `Tkinter`/`tkinter` is put in a separate output. The `tkinter` is available as `python35Packages.tkinter`.
+To reduce closure size the `Tkinter`/`tkinter` is available as a separate package, `pythonPackages.tkinter`.
 
 #### Attributes on interpreters packages