bpo-33729: Fix issues with arguments parsing in hashlib. (GH-8346)

* help(hashlib) didn't work because of incorrect module name in blake2b and
  blake2s classes.
* Constructors blake2*(), sha3_*(), shake_*() and keccak_*() incorrectly
  accepted keyword argument "string" for binary data, but documented as
  accepting the "data" keyword argument. Now this parameter is positional-only.
* Keyword-only parameters in blake2b() and blake2s() were not documented as
  keyword-only.
* Default value for some parameters of blake2b() and blake2s() was None,
  which is not acceptable value.
* The length argument for shake_*.digest() was wrapped out to 32 bits.
* The argument for shake_128.digest() and shake_128.hexdigest() was not
  positional-only as intended.
* TypeError messages for incorrect arguments in all constructors sha3_*(),
  shake_*() and keccak_*() incorrectly referred to sha3_224.

Also made the following enhancements:

* More accurately specified input and result types for strings, bytes and
  bytes-like objects.
* Unified positional parameter names for update() and constructors.
* Improved formatting.

Lib/hashlib.py

@@ -25,18 +25,18 @@ Choose your hash function wisely.  Some have known collision weaknesses.
 sha384 and sha512 will be slow on 32 bit platforms.
 
 Hash objects have these methods:
- - update(arg): Update the hash object with the bytes in arg. Repeated calls
-                are equivalent to a single call with the concatenation of all
-                the arguments.
- - digest():    Return the digest of the bytes passed to the update() method
-                so far.
- - hexdigest(): Like digest() except the digest is returned as a unicode
-                object of double length, containing only hexadecimal digits.
- - copy():      Return a copy (clone) of the hash object. This can be used to
-                efficiently compute the digests of strings that share a common
-                initial substring.
+ - update(data): Update the hash object with the bytes in data. Repeated calls
+                 are equivalent to a single call with the concatenation of all
+                 the arguments.
+ - digest():     Return the digest of the bytes passed to the update() method
+                 so far as a bytes object.
+ - hexdigest():  Like digest() except the digest is returned as a string
+                 of double length, containing only hexadecimal digits.
+ - copy():       Return a copy (clone) of the hash object. This can be used to
+                 efficiently compute the digests of datas that share a common
+                 initial substring.
 
-For example, to obtain the digest of the string 'Nobody inspects the
+For example, to obtain the digest of the byte string 'Nobody inspects the
 spammish repetition':
 
     >>> import hashlib
@@ -130,14 +130,15 @@ def __get_openssl_constructor(name):
 
 def __py_new(name, data=b'', **kwargs):
     """new(name, data=b'', **kwargs) - Return a new hashing object using the
-    named algorithm; optionally initialized with data (which must be bytes).
+    named algorithm; optionally initialized with data (which must be
+    a bytes-like object).
     """
     return __get_builtin_constructor(name)(data, **kwargs)
 
 
 def __hash_new(name, data=b'', **kwargs):
     """new(name, data=b'') - Return a new hashing object using the named algorithm;
-    optionally initialized with data (which must be bytes).
+    optionally initialized with data (which must be a bytes-like object).
     """
     if name in {'blake2b', 'blake2s'}:
         # Prefer our blake2 implementation.