fix(win_mmap): unmark hidden win_errors due to smmap unicode foes

Now 2 more TCs pass in Windows:
+ TestRepo.test_file_handle_leaks()
+ TestObjDbPerformance.test_random_access()

See gitpython-developers/smmap#30

Author: ankostis

Diff for: git/test/performance/test_odb.py

@@ -3,10 +3,6 @@
 
 import sys
 from time import time
-from unittest.case import skipIf
-
-from git.compat import PY3
-from git.util import HIDE_WINDOWS_KNOWN_ERRORS
 
 from .lib import (
     TestBigRepoR
@@ -15,8 +11,6 @@
 
 class TestObjDBPerformance(TestBigRepoR):
 
-    @skipIf(HIDE_WINDOWS_KNOWN_ERRORS and PY3,
-            "FIXME: smmp fails with: TypeError: Can't convert 'bytes' object to str implicitly")
     def test_random_access(self):
         results = [["Iterate Commits"], ["Iterate Blobs"], ["Retrieve Blob Data"]]
         for repo in (self.gitrorepo, self.puregitrorepo):

Diff for: git/test/test_repo.py

@@ -808,8 +808,6 @@ def test_git_file(self, rwrepo):
         git_file_repo = Repo(rwrepo.working_tree_dir)
         self.assertEqual(osp.abspath(git_file_repo.git_dir), real_path_abs)
 
-    @skipIf(HIDE_WINDOWS_KNOWN_ERRORS and PY3,
-            "FIXME: smmp fails with: TypeError: Can't convert 'bytes' object to str implicitly")
     def test_file_handle_leaks(self):
         def last_commit(repo, rev, path):
             commit = next(repo.iter_commits(rev, path, max_count=1))

Diff for: git/test/test_submodule.py

@@ -9,7 +9,6 @@
 from git.cmd import Git
 from git.compat import (
     string_types,
-    is_win,
 )
 from git.exc import (
     InvalidGitRepositoryError,