Detect whether a method is abstract or always raise an exception

[tristanlatr]

Abstract methods and methods unconditionally raising exceptions should be handled specially because
we should not invite developers to use them directly.
This can be done by adding a special note for instance.

Also, these methods should not be shown in the constructors list on the header of class pages.

Examples of method that should be considered abstract:

# pydoctor should not care how the meta class is added since 
# it has no effect without decorated functions and the decorator expression is all new need.
class C: # Explicitly abstract class
	# Explicitly abstract methods
	import abc
	@abc.abstractmethod
	def f(): 
	@abc.abstractproperty
	def g(): 
	
	# Methods that does not do anything
	def h(): 
		"docs"
		...
	def i(): 
		"docs"
	def j(): ...
	def k(): pass
	
	# Methods that always raise the NotImplementedError
	def l(): raise NotImplementedError
	def m(): raise NotImplementedError()

class B(C):
	# Implicitely abstract class since the explicitly needed method "g"
	# is not implemented
	f,_ = *iter([)

class D(C):
	# Concrete class! Even if the "I" is missing, it's not an
	# explicitly needed method so pydoctor keeps it conservative.
	f,g,h,_,j,k,l,m = *iter([])
	

Mypy implemementation: https://github.com/python/mypy/blob/v1.13.0/mypy/semanal.py#L7550
We can do roughly the same.

Other interesting mypy code fragments:

https://github.com/python/mypy/blob/52888aec43ef8ba59645c7cd3ff5725ff9a861d7/mypy/checkexpr.py#L6234

https://github.com/python/mypy/blob/52888aec43ef8ba59645c7cd3ff5725ff9a861d7/mypy/semanal_classprop.py#L42

[tristanlatr]

Also instead of showing « overridden in » we should say « implemented in »