beartype.door API tests x 5.

This commit is the next in a commit chain repairing currently failing
tests pertaining to the `beartype.door` subpackage, recently implemented
by Harvard microscopist and general genius @tlambert03 in PR #136.
Specifically, this commit:

* Declares a new `beartype.roar.BeartypeDoorNonpepException` type,
  raised when the `beartype.roar.TypeHint` constructor is passed an
  object that is *not* a PEP-compliant type hint currently supported by
  the DOOR API.
* Extricates the `beartype.door._doorcls.is_subclass()` and
  `beartype.door._doorcls.die_unless_typehint()` functions into a new
  `beartype.door._doortest` submodule for maintainability.
* Folds the prior `test_typehint_fail()` unit test into the existing
  `test_typehint_new()` unit test.

(*Thunderous blunderbuss!*)

beartype/door/__init__.py

@@ -4,7 +4,7 @@
 # See "LICENSE" for further details.
 
 '''
-**Beartype Decidedly Object-Orientedly Recursive (DOOR) API.**
+**Beartype Decidedly Object-Oriented Runtime-checking (DOOR) API.**
 
 This subpackage provides an object-oriented type hint class hierarchy,
 encapsulating the crude non-object-oriented type hint declarative API
@@ -23,5 +23,7 @@
 #!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 from beartype.door._doorcls import (
     TypeHint as TypeHint,
+)
+from beartype.door._doortest import (
     is_subhint as is_subhint,
 )