Allow TypeVar names to start with underscores

This is consistent with our CapWords-based N801 class name check, which
shares a similar PEP 8 definition.

Resolves #245

Author: jparise

src/pep8ext_naming.py

@@ -537,13 +537,14 @@ def visit_module(self, node, parents: Sequence, ignored: NameSet):
             if func_name != "TypeVar" or name in ignored:
                 continue
 
-            if len(args) == 0 or args[0] != name:
+            if not args or args[0] != name:
                 yield self.err(body, 'N808', name=name)
 
-            if not name[:1].isupper():
+            stripped_name = name.lstrip('_')
+            if not stripped_name[:1].isupper():
                 yield self.err(body, 'N808', name=name)
 
-            parts = name.split('_')
+            parts = stripped_name.split('_')
             if len(parts) > 2:
                 yield self.err(body, 'N808', name=name)
 

testsuite/N808.py

@@ -42,8 +42,8 @@
 #: Okay
 Good = TypeVar("Good")
 
-#: N808
-__NotGood = TypeVar("__NotGood")
+#: Okay
+__Good = TypeVar("__Good")
 
 #: N808
 __NotGood__ = TypeVar("__NotGood__")