fix: mypy plugin avoid calling get_typename on None

[timleslie]

Upgrading to the latest version (0.11.0 -> 0.14.5) I got the following error:

(.venv) ➜  xxxx git:(chore/upgrade-pandera) mypy src --show-traceback
xxxxxxx.py:150: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.1.1
Traceback (most recent call last):
  File "mypy/semanal.py", line 6261, in accept
  File "mypy/nodes.py", line 1123, in accept
  File "mypy/semanal.py", line 1561, in visit_class_def
  File "mypy/semanal.py", line 1641, in analyze_class
  File "mypy/semanal.py", line 1668, in analyze_class_body_common
  File "mypy/semanal.py", line 1751, in apply_class_plugin_hooks
  File "/xxxxxxx/.venv/lib/python3.8/site-packages/pandera/mypy.py", line 93, in _pandera_model_class_maker_callback
    transformer.transform()
  File "/xxxxxxx/.venv/lib/python3.8/site-packages/pandera/mypy.py", line 125, in transform
    self.erase_field_type_arg()
  File "/xxxxxxx/.venv/lib/python3.8/site-packages/pandera/mypy.py", line 143, in erase_field_type_arg
    if get_typename(def_.type) in FIELD_GENERICS_FULLNAMES:
  File "/xxxxxxx.venv/lib/python3.8/site-packages/pandera/mypy.py", line 164, in get_typename
    return f"{x.type.module_name}.{x.type.name}"
AttributeError: 'NoneType' object has no attribute 'type'
xxxxxxx.py:150: : note: use --pdb to drop into pdb

This was due to model classes that included @staticmethod and @classmethod methods on the model. These came through with def_.type being None.

This PR filters these methods out when calling erase_field_type_arg.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (285f4b9) 97.46% compared to head (403dbb2) 97.46%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1154   +/-   ##
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  Coverage   97.46%   97.46%           
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  Files          67       67           
  Lines        4844     4844           
=======================================
  Hits         4721     4721           
  Misses        123      123           

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[cosmicBboy]

thanks @timleslie !