monkeypatch os.path.exists over less code

PipSession attempts to create a user_agent which results in calls to
platform.linux_distribution being called, there is a chance this hits
platform._dist_try_harder, which calls os.path.exists("/a/bad/path")
then attempts to open and read from said path resulting in brekage.

This creates PipSession before mock patches os.path.exists then only
patches the call that requires os.path.exists to be patched.

tests/unit/test_finder.py

@@ -34,13 +34,11 @@ def test_no_partial_name_match(data):
     assert found.url.endswith("gmpy-1.15.tar.gz"), found
 
 
-@patch(
-    'pip.index.os.path.exists',
-    return_value=True  # b/c we only use tilde expanded version if it exists
-)
 def test_tilde(data):
     """Finder can accept a path with ~ in it and will normalize it."""
-    finder = PackageFinder(['~/python-pkgs'], [], session=PipSession())
+    session = PipSession()
+    with patch('pip.index.os.path.exists', return_value=True):
+        finder = PackageFinder(['~/python-pkgs'], [], session=session)
     req = InstallRequirement.from_line("gmpy")
     with pytest.raises(DistributionNotFound):
         finder.find_requirement(req, False)