fix some issues found by stubcheck (#1303)

- ThreadError exists (undocumented) on Python 3. It's an alias for _thread.error,
  but making it a separate exception seems fine.
- zipfile.error is an alias for BadZipFile on both Python versions.
- zlib.Compress and Decompress are not actually accessible at runtime.

stdlib/2and3/threading.pyi

@@ -37,8 +37,7 @@ def stack_size(size: int = ...) -> int: ...
 if sys.version_info >= (3,):
     TIMEOUT_MAX = ...  # type: int
 
-if sys.version_info < (3,):
-    class ThreadError(Exception): ...
+class ThreadError(Exception): ...
 
 
 # TODO: Change to a class with __getattr__ and __setattr__

stdlib/2and3/zipfile.pyi

@@ -14,6 +14,7 @@ if sys.version_info >= (3,):
     BadZipfile = BadZipFile
 else:
     class BadZipfile(Exception): ...
+error = BadZipfile
 
 class LargeZipFile(Exception): ...
 

stdlib/2and3/zlib.pyi

@@ -22,20 +22,20 @@ if sys.version_info >= (3,):
 class error(Exception): ...
 
 
-class Compress:
+class _Compress:
     def compress(self, data: bytes) -> bytes: ...
     def flush(self, mode: int = ...) -> bytes: ...
-    def copy(self) -> "Compress": ...
+    def copy(self) -> _Compress: ...
 
 
-class Decompress:
+class _Decompress:
     unused_data = ...  # type: bytes
     unconsumed_tail = ...  # type: bytes
     if sys.version_info >= (3,):
         eof = ...  # type: bool
     def decompress(self, data: bytes, max_length: int = ...) -> bytes: ...
     def flush(self, length: int = ...) -> bytes: ...
-    def copy(self) -> "Decompress": ...
+    def copy(self) -> _Decompress: ...
 
 
 def adler32(data: bytes, value: int = ...) -> int: ...
@@ -43,13 +43,13 @@ def compress(data: bytes, level: int = ...) -> bytes: ...
 if sys.version_info >= (3,):
     def compressobj(level: int = ..., method: int = ..., wbits: int = ...,
                     memLevel: int = ..., strategy: int = ...,
-                    zdict: bytes = ...) -> Compress: ...
+                    zdict: bytes = ...) -> _Compress: ...
 else:
     def compressobj(level: int = ..., method: int = ..., wbits: int = ...,
-                    memlevel: int = ..., strategy: int = ...) -> Compress: ...
+                    memlevel: int = ..., strategy: int = ...) -> _Compress: ...
 def crc32(data: bytes, value: int = ...) -> int: ...
 def decompress(data: bytes, wbits: int = ..., bufsize: int = ...) -> bytes: ...
 if sys.version_info >= (3,):
-    def decompressobj(wbits: int = ..., zdict: bytes = ...) -> Decompress: ...
+    def decompressobj(wbits: int = ..., zdict: bytes = ...) -> _Decompress: ...
 else:
-    def decompressobj(wbits: int = ...) -> Decompress: ...
+    def decompressobj(wbits: int = ...) -> _Decompress: ...