Refactor: Fix pylint code quality issues across adiftools package (9.22 → 9.95/10)

[Copilot]

Several code quality issues identified by pylint across the package — incorrect exception syntax, missing file encodings, non-idiomatic type checks, and style issues in the graph module.

adiftools.py

• Consolidated 4 fragmented try/except import blocks into one; fixed binary-or exception syntax: except (ModuleNotFoundError, ImportError)
• Added encoding='utf-8' to all open() calls
• Simplified is_loaded property to return len(self.df_adif) > 0
• Initialized _temp_dfs in __init__; removed hasattr guards in read_adi_streaming

callsign.py / gridlocator.py

• type(x) is not str → not isinstance(x, str)

adifgraph.py

• except ModuleNotFoundError or ImportError → except (ModuleNotFoundError, ImportError)
• Renamed bar (disallowed name) → bars; unused fig → _
• '%d' % int(height) → f'{int(height):d}'
• Moved from pandas.api.types import PeriodDtype and import matplotlib as mpl re-import to module top-level
• sum([row[band] for band in bands]) → generator expression
• Hoisted text_rotation calculation out of loop in monthly_band_qso

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