Add support for type variable name convention check

[itaihay]

Proposed implementation for #163.

Adds support for type variable name convention check, as mentioned in PEP8:

Names of type variables introduced in PEP 484 should normally use CapWords preferring short names: T, AnyStr, Num. It is recommended to add suffixes _co or _contra to the variables used to declare covariant or contravariant behavior correspondingly:

Proposed name is N808 due to the relationship with the class name convention.

It's my first time contributing, so please let me know if anything needs to be done differently!
Thank you

[jparise]

Thanks for the contribution!

Do you think we should consider the TypeVar's arguments? Otherwise, we can only check if the given name matches one of the allowed conventions, but we can't check if the name matches the TypeVar's specific values.

For example, would we want these mismatched forms to raise warnings?

• Var = TypeVar("Var", covariant=True)
• Var_co = TypeVar("Var", contravariant=True)
• Var_contra = TypeVar("Var", covariant=True)

[jparise]

As a small optimization, let's check the cheaper expression first:

Suggested change

	if _ignored(name, ignore) or func_name != "TypeVar":
	if func_name != "TypeVar" or _ignored(name, ignore):

[jparise]

I think this might be a little clearer if it focuses on a single suffix value:

Suggested change

	underscore_split = name.split('_')
	suffix = underscore_split[-1]
	if len(underscore_split) > 2 or (suffix != 'co' and suffix != 'contra'):
	parts = name.split('_')
	suffix = parts[-1] if len(parts) > 2 else ''
	if suffix and suffix != 'co' and suffix != 'contra':