test: add test to try importing every file

The testsuite does not currently exercise every file, but we can at
least try importing it to make sure it isn't obviously broken. This
additionally helps to iterate through python3 syntax-level
incompatibilities.

src/calibre/test_build.py

@@ -73,6 +73,27 @@ class BuildTest(unittest.TestCase):
         from html5_parser import parse
         parse('<p>xxx')
 
+    def test_imports(self):
+        import importlib
+        exclude = ['dbus_export.demo', 'dbus_export.gtk',  'upstream']
+        if not iswindows:
+            exclude.extend(['iphlpapi', 'windows', 'winreg', 'winusb'])
+        if not isosx:
+            exclude.append('osx')
+        if not islinux:
+            exclude.extend(['dbus', 'linux'])
+        for root, dirs, files in os.walk('src/calibre'):
+            for dir in dirs:
+                if not os.path.isfile(os.path.join(root, dir, '__init__.py')):
+                    dirs.remove(dir)
+            for file in files:
+                file, ext = os.path.splitext(file)
+                if ext != '.py':
+                    continue
+                name = '.'.join(root.split(os.path.sep)[1:] + [file])
+                if not any(x for x in exclude if x in name):
+                    importlib.import_module(name)
+
     def test_plugins(self):
         exclusions = set()
         if is_ci: