Fix resource leak in WheelFile.open() (#338)

In WheelFile.open(), if the hash does not exist, avoid opening the file
before raising the exception. Previously would leak the open file
resource which could result in a ResourceWarning when Python warnings
are enabled:

    ResourceWarning: unclosed file <_io.FileIO name='…/test_testzip_missing_hash0/test-1.0-py2.py3-none-any.whl' mode='rb' closefd=True>

Python warnings are now enabled during tests to help catch these
earlier.

.github/workflows/codeqa-test.yml

@@ -50,4 +50,4 @@ jobs:
     - name: Install test dependencies
       run: pip install .[test]
     - name: Test with pytest
-      run: pytest
+      run: python -b -m pytest -W always

src/wheel/wheelfile.py

@@ -90,13 +90,13 @@ def _update_crc(newdata, eof=None):
             if eof and running_hash.digest() != expected_hash:
                 raise WheelError("Hash mismatch for file '{}'".format(native(ef_name)))
 
-        ef = ZipFile.open(self, name_or_info, mode, pwd)
         ef_name = as_unicode(name_or_info.filename if isinstance(name_or_info, ZipInfo)
                              else name_or_info)
-        if mode == 'r' and not ef_name.endswith('/'):
-            if ef_name not in self._file_hashes:
-                raise WheelError("No hash found for file '{}'".format(native(ef_name)))
+        if mode == 'r' and not ef_name.endswith('/') and ef_name not in self._file_hashes:
+            raise WheelError("No hash found for file '{}'".format(native(ef_name)))
 
+        ef = ZipFile.open(self, name_or_info, mode, pwd)
+        if mode == 'r' and not ef_name.endswith('/'):
             algorithm, expected_hash = self._file_hashes[ef_name]
             if expected_hash is not None:
                 # Monkey patch the _update_crc method to also check for the hash from RECORD

tox.ini

@@ -9,7 +9,7 @@ minversion = 3.3.0
 skip_missing_interpreters = true
 
 [testenv]
-commands = pytest {posargs}
+commands = {envpython} -b -m pytest -W always {posargs}
 extras = test
 
 [testenv:flake8]