docker: add python stdlib dependency (required by keycodemapdb)

Since 92712822 QEMU depends of keycodemapdb, which uses the python 'csv'
module from stdlib to parse keymaps.csv.

Without this package the build fails:

    GEN     ui/input-keymap-linux-to-qcode.c
  Traceback (most recent call last):
    File "ui/keycodemapdb/tools/keymap-gen", line 15, in <module>
      import csv
  ImportError: No module named csv
    GEN     ui/input-keymap-qcode-to-qnum.c
  Traceback (most recent call last):
    File "ui/keycodemapdb/tools/keymap-gen", line 15, in <module>
      import csv
  ImportError: No module named csv
  [...]
    CC      ui/input-keymap.o
  ui/input-keymap.c:8:44: fatal error: ui/input-keymap-linux-to-qcode.c: No such file or directory
  make: *** [ui/input-keymap.o] Error 1

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>