refactor: we no longer need to protect against UnicodeError, it was o…

…nly on 3.6

This was the failure it was protecting against on Python 3.6:

```
  ___________________________ ProcessTest.test_lang_c ____________________________
  [gw0] linux -- Python 3.6.15 /home/runner/work/coveragepy/coveragepy/.tox/py36/bin/python

  self = <tests.test_process.ProcessTest object at 0x7fe57e63b198>

      @pytest.mark.skipif(env.PYPY, reason="PyPy is unreliable with this test")
      # Jython as of 2.7.1rc3 won't compile a filename that isn't utf-8.
      @pytest.mark.skipif(env.JYTHON, reason="Jython can't handle this test")
      def test_lang_c(self):
          # LANG=C forces getfilesystemencoding on Linux to 'ascii', which causes
          # failures with non-ascii file names. We don't want to make a real file
          # with strange characters, though, because that gets the test runners
          # tangled up.  This will isolate the concerns to the coverage.py code.
          # #533
          self.make_file("weird_file.py", r"""
              globs = {}
              code = "a = 1\nb = 2\n"
              exec(compile(code, "wut\xe9\xea\xeb\xec\x01\x02.py", 'exec'), globs)
              print(globs['a'])
              print(globs['b'])
              """)
          self.set_environ("LANG", "C")
          out = self.run_command("coverage run weird_file.py")
  >       assert out == "1\n2\n"
  E       assert 'Traceback (m...ion by zero\n' == '1\n2\n'
  E         - 1
  E         - 2
  E         + Traceback (most recent call last):
  E         +   File "/home/runner/work/coveragepy/coveragepy/coverage/files.py", line 149, in abs_file
  E         +     path = os.path.realpath(path)
  E         +   File "/opt/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/posixpath.py", line 395, in realpath
  E         +     path, ok = _joinrealpath(filename[:0], filename, {})
  E         +   File "/opt/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/posixpath.py", line 429, in _joinrealpath
  E         +     if not islink(newpath):
  E         +   File "/opt/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/posixpath.py", line 171, in islink
  E         +     st = os.lstat(path)
  E         + UnicodeEncodeError: 'ascii' codec can't encode characters in position 3-6: ordinal not in range(128)
```

coverage/files.py

@@ -145,13 +145,7 @@ def actual_path(path):
 @contract(returns='unicode')
 def abs_file(path):
     """Return the absolute normalized form of `path`."""
-    try:
-        path = os.path.realpath(path)
-    except UnicodeError:
-        pass
-    path = os.path.abspath(path)
-    path = actual_path(path)
-    return path
+    return actual_path(os.path.abspath(os.path.realpath(path)))
 
 
 def python_reported_file(filename):