Revert "refactor setup.py because 'build' wants to be a pissy whinybaby"

This reverts commit 1f7f85df93.

requirements.txt

@@ -0,0 +1,3 @@
+discord.py>=2.0.0
+aiohttp>=3.7.4,<4
+orjson

setup.py

@@ -2,7 +2,6 @@ import setuptools
 import re
 
 version = ''
-requirements = ['discord.py>=2.0.0', 'aiohttp>=3.7.4,<4', 'orjson']
 with open('pomice/__init__.py') as f:
     version = re.search(r'^__version__\s*=\s*[\'"]([^\'"]*)[\'"]', f.read(), re.MULTILINE).group(1)
 
@@ -26,7 +25,8 @@ if version.endswith(('a', 'b', 'rc')):
     except Exception:
         pass
 
-
+with open("requirements.txt") as f:
+    requirements = f.read().splitlines()
 
 with open("README.md") as f:
     readme = f.read()