pipでライブラリインストール時に VersionConflict、DistributionNotFound 発生。暫定対応。

pipでライブラリのインストールを実行した際にエラーが発生しました。
原因は分からないままですが、インストールできたのでメモ。

#エラー内容

※Ansibleを使用して実行していた際に発生したエラーメッセージの一部のため、通常とは違う可能性があります。

```bash:エラーメッセージ
"msg": 
:stderr: Traceback (most recent call last):
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 574, in _build_master
    ws.require(__requires__)
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 892, in require
    needed = self.resolve(parse_requirements(requirements))
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 783, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.VersionConflict: (pip 10.0.1 (/usr/local/lib/python3.6/site-packages), Requirement.parse('pip==9.0.3'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File \"/usr/bin/pip-3.6\", line 6, in <module>
    from pkg_resources import load_entry_point
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 3088, in <module>
    @_call_aside
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 3072, in _call_aside
    f(*args, **kwargs)
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 3101, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 576, in _build_master
    return cls._build_from_requirements(__requires__)
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 589, in _build_from_requirements
    dists = ws.resolve(reqs, Environment())
  File \"/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py\", line 778, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'pip==9.0.3' distribution was not found and is required by the application

```

#暫定対応
`/usr/bin/pip-3.6`ではなく、`/usr/local/bin/pip3.6`を使用する。

追記(20190919)：
　`/usr/local/bin/pip3.6`を使用すると逆にコマンドがないとエラーになるようになりました。意味不明・・。
　コマンドがないとエラーになった場合は、下記補足のようにコマンドを確認するのが良さそうです。

#補足
```bash:補足ログ
$ which pip-3.6
/usr/bin/pip-3.6

$ which pip3.6
/usr/local/bin/pip3.6

$ pip-3.6 -V
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 574, in _build_master
    ws.require(__requires__)
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 892, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 783, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.VersionConflict: (pip 10.0.1 (/usr/local/lib/python3.6/site-packages), Requirement.parse('pip==9.0.3'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/pip-3.6", line 6, in <module>
    from pkg_resources import load_entry_point
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3088, in <module>
    @_call_aside
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3072, in _call_aside
    f(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3101, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 576, in _build_master
    return cls._build_from_requirements(__requires__)
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 589, in _build_from_requirements
    dists = ws.resolve(reqs, Environment())
  File "/usr/local/lib/python3.6/site-packages/pkg_resources/__init__.py", line 778, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'pip==9.0.3' distribution was not found and is required by the application

$ pip3.6 -V
pip 10.0.1 from /usr/local/lib/python3.6/site-packages/pip (python 3.6)

```

なぜ2つあるのかも正直謎です。
入れたことを忘れていたのか？

自分の環境のみで偶然発生した事象かもしれませんが、参考になれば幸いです。

以上です。