Explicitly declare units field in PintType

Improve mypy compatibility by declaring `units` to be a field of PintType.

Signed-off-by: Michael Tiemann <72577720+MichaelTiemannOSC@users.noreply.github.com>

pint_pandas/pint_array.py

@@ -2,6 +2,7 @@
 import re
 import warnings
 from importlib.metadata import version
+from typing import Optional
 
 import numpy as np
 import pandas as pd
@@ -43,6 +44,7 @@ class PintType(ExtensionDtype):
     # str = '|O08'
     # base = np.dtype('O')
     # num = 102
+    units: Optional[_Unit] = None  # Filled in by `construct_from_..._string`
     _metadata = ("units",)
     _match = re.compile(r"(P|p)int\[(?P<units>.+)\]")
     _cache = {}