Refactor concurrency so that it is pluggable

However, this may not be the best idea because it involves global
state that could be overridden by a parallel call to ocrmypdf.ocr.

src/ocrmypdf/_plugin_manager.py

@@ -32,7 +32,11 @@ class OcrmypdfPluginManager(pluggy.PluginManager):
     """
 
     def __init__(
-        self, *args, plugins: List[Union[str, Path]], builtins: bool = True, **kwargs,
+        self,
+        *args,
+        plugins: List[Union[str, Path]],
+        builtins: bool = True,
+        **kwargs,
     ):
         self.__init_args = args
         self.__init_kwargs = kwargs
@@ -88,7 +92,9 @@ class OcrmypdfPluginManager(pluggy.PluginManager):
 
 def get_plugin_manager(plugins: List[Union[str, Path]], builtins=True):
     pm = OcrmypdfPluginManager(
-        project_name='ocrmypdf', plugins=plugins, builtins=builtins,
+        project_name='ocrmypdf',
+        plugins=plugins,
+        builtins=builtins,
     )
     return pm