setup.py: remove pip import

.github/workflows/linux.yml

@@ -3,6 +3,9 @@ on:
   push:
     branches:
       - master
+  pull_request:
+    branches:
+      - '**'
 
 jobs:
   main:

setup.py

@@ -1,15 +1,7 @@
 #!/usr/bin/env python
 # coding: utf-8
 
-# Pip Package Manager
-# ------------------------------------------------------------------------------
-try:
-    import pip
-    import setuptools
-except ImportError:
-    url = "http://pip.readthedocs.org"
-    error = f"pip is not installed, refer to <{url}> for instructions."
-    raise ImportError(error)
+import setuptools
 
 # Pandoc Metadata
 # ------------------------------------------------------------------------------