matplotlibのインストール

$ pip3 install matplotlib
Requirement already satisfied: matplotlib in /home/vorac/.local/lib/python3.7/site-packages (3.0.3)
Requirement already satisfied: cycler>=0.10 in /home/vorac/.local/lib/python3.7/site-packages (from matplotlib) (0.10.0)
Requirement already satisfied: python-dateutil>=2.1 in /home/vorac/.local/lib/python3.7/site-packages (from matplotlib) (2.8.0)
Requirement already satisfied: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in /home/vorac/.local/lib/python3.7/site-packages (from matplotlib) (2.3.0)
Requirement already satisfied: numpy>=1.10.0 in /home/vorac/.local/lib/python3.7/site-packages (from matplotlib) (1.16.3)
Requirement already satisfied: kiwisolver>=1.0.1 in /home/vorac/.local/lib/python3.7/site-packages (from matplotlib) (1.1.0)
Requirement already satisfied: six in /usr/lib/python3/dist-packages (from cycler>=0.10->matplotlib) (1.12.0)
Requirement already satisfied: setuptools in /usr/lib/python3/dist-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)

問題ない今：

$ python3
Python 3.7.3rc1 (default, Mar 13 2019, 11:01:15) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
>>> 
>>> 
>>> import matplotlib.pyplot as plt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vorac/.local/lib/python3.7/site-packages/matplotlib/pyplot.py", line 2372, in <module>
    switch_backend(rcParams["backend"])
  File "/home/vorac/.local/lib/python3.7/site-packages/matplotlib/pyplot.py", line 207, in switch_backend
    backend_mod = importlib.import_module(backend_name)
  File "/usr/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/home/vorac/.local/lib/python3.7/site-packages/matplotlib/backends/backend_tkagg.py", line 1, in <module>
    from . import _backend_tk
  File "/home/vorac/.local/lib/python3.7/site-packages/matplotlib/backends/_backend_tk.py", line 5, in <module>
    import tkinter as Tk
ModuleNotFoundError: No module named 'tkinter'

奇妙な。

$ pip3 install tkinter
Collecting tkinter
Could not install packages due to an EnvironmentError: 404 Client Error: Not Found for url: https://pypi.org/simple/tkinter/

ページには実際には「404 Not Found」というテキストのみが含まれています。この状況ではどうすればよいですか？ローカル$ HOMEにパッケージをインストールしようとしていますが、システム全体のインストールも可能です。