bpo-40275: More lazy imports in test.support (GH-20131)

Make the the following imports lazy in test.support:

* bz2
* gzip
* lzma
* resource
* zlib

The following test.support decorators now need to be called
with parenthesis:

* @support.requires_bz2
* @support.requires_gzip
* @support.requires_lzma
* @support.requires_zlib

For example, "@requires_zlib" becomes "@requires_zlib()".

Lib/test/support/__init__.py

@@ -28,31 +28,6 @@ import warnings
 
 from .testresult import get_test_runner
 
-try:
-    import zlib
-except ImportError:
-    zlib = None
-
-try:
-    import gzip
-except ImportError:
-    gzip = None
-
-try:
-    import bz2
-except ImportError:
-    bz2 = None
-
-try:
-    import lzma
-except ImportError:
-    lzma = None
-
-try:
-    import resource
-except ImportError:
-    resource = None
-
 __all__ = [
     # globals
     "PIPE_MAX_SIZE", "verbose", "max_memuse", "use_resources", "failfast",
@@ -705,13 +680,33 @@ requires_IEEE_754 = unittest.skipUnless(
     float.__getformat__("double").startswith("IEEE"),
     "test requires IEEE 754 doubles")
 
-requires_zlib = unittest.skipUnless(zlib, 'requires zlib')
+def requires_zlib(reason='requires zlib'):
+    try:
+        import zlib
+    except ImportError:
+        zlib = None
+    return unittest.skipUnless(zlib, reason)
 
-requires_gzip = unittest.skipUnless(gzip, 'requires gzip')
+def requires_gzip(reason='requires gzip'):
+    try:
+        import gzip
+    except ImportError:
+        gzip = None
+    return unittest.skipUnless(gzip, reason)
 
-requires_bz2 = unittest.skipUnless(bz2, 'requires bz2')
+def requires_bz2(reason='requires bz2'):
+    try:
+        import bz2
+    except ImportError:
+        bz2 = None
+    return unittest.skipUnless(bz2, reason)
 
-requires_lzma = unittest.skipUnless(lzma, 'requires lzma')
+def requires_lzma(reason='requires lzma'):
+    try:
+        import lzma
+    except ImportError:
+        lzma = None
+    return unittest.skipUnless(lzma, reason)
 
 is_jython = sys.platform.startswith('java')
 
@@ -1062,6 +1057,10 @@ def check_syntax_warning(testcase, statement, errtext='', *, lineno=1, offset=No
 
 def open_urlresource(url, *args, **kw):
     import urllib.request, urllib.parse
+    try:
+        import gzip
+    except ImportError:
+        gzip = None
 
     check = kw.pop('check', None)
 
@@ -2579,11 +2578,16 @@ class SuppressCrashReport:
                     self.old_modes[report_type] = old_mode, old_file
 
         else:
-            if resource is not None:
+            try:
+                import resource
+                self.resource = resource
+            except ImportError:
+                self.resource = None
+            if self.resource is not None:
                 try:
-                    self.old_value = resource.getrlimit(resource.RLIMIT_CORE)
-                    resource.setrlimit(resource.RLIMIT_CORE,
-                                       (0, self.old_value[1]))
+                    self.old_value = self.resource.getrlimit(self.resource.RLIMIT_CORE)
+                    self.resource.setrlimit(self.resource.RLIMIT_CORE,
+                                            (0, self.old_value[1]))
                 except (ValueError, OSError):
                     pass
 
@@ -2621,9 +2625,9 @@ class SuppressCrashReport:
                     msvcrt.CrtSetReportMode(report_type, old_mode)
                     msvcrt.CrtSetReportFile(report_type, old_file)
         else:
-            if resource is not None:
+            if self.resource is not None:
                 try:
-                    resource.setrlimit(resource.RLIMIT_CORE, self.old_value)
+                    self.resource.setrlimit(self.resource.RLIMIT_CORE, self.old_value)
                 except (ValueError, OSError):
                     pass