Streamline setup.py file reading

This is partly a refactoring, reducing logic duplication and
slightly decreasing overall length of the code that reads values
from files. However, it does make two small behavioral changes:

- In addition to the VERSION file for which this was already the
  case, requirements files and the readme are also taken relative
  to the directory that contains the setup.py file, rather than
  relative to the current directory if different. Supporting
  reading the readme and dependencies realtive to a different
  directory doesn't seem intended, and could lead to confusion in
  some cases.

- The VERSION file is now read entirely and whitespace stripped,
  rather than reading just the first line. The VERSION file should
  always contain exactly one line, and this logic is simpler.

This changes code for the reading of metadata only. It does not
make any changes to the version stamping code.

setup.py

@@ -2,24 +2,22 @@
 
 import os
 import sys
+from pathlib import Path
 from typing import Sequence
 
 from setuptools import setup, find_packages
 from setuptools.command.build_py import build_py as _build_py
 from setuptools.command.sdist import sdist as _sdist
 
 
-with open(os.path.join(os.path.dirname(__file__), "VERSION"), encoding="utf-8") as ver_file:
-    VERSION = ver_file.readline().strip()
+def _read_content(path: str) -> str:
+    return (Path(__file__).parent / path).read_text(encoding="utf-8")
 
-with open("requirements.txt", encoding="utf-8") as reqs_file:
-    requirements = reqs_file.read().splitlines()
 
-with open("test-requirements.txt", encoding="utf-8") as reqs_file:
-    test_requirements = reqs_file.read().splitlines()
-
-with open("README.md", encoding="utf-8") as rm_file:
-    long_description = rm_file.read()
+version = _read_content("VERSION").strip()
+requirements = _read_content("requirements.txt").splitlines()
+test_requirements = _read_content("test-requirements.txt").splitlines()
+long_description = _read_content("README.md")
 
 
 class build_py(_build_py):
@@ -50,7 +48,7 @@ def _stamp_version(filename: str) -> None:
         with open(filename) as f:
             for line in f:
                 if "__version__ =" in line:
-                    line = line.replace('"git"', "'%s'" % VERSION)
+                    line = line.replace('"git"', "'%s'" % version)
                     found = True
                 out.append(line)
     except OSError:
@@ -66,7 +64,7 @@ def _stamp_version(filename: str) -> None:
 setup(
     name="GitPython",
     cmdclass={"build_py": build_py, "sdist": sdist},
-    version=VERSION,
+    version=version,
     description="GitPython is a Python library used to interact with Git repositories",
     author="Sebastian Thiel, Michael Trier",
     author_email="byronimo@gmail.com, mtrier@gmail.com",