Use mypy default follow import strategy

[WillAyd]

This was put in place as we were fixing up old annotations but isn't required at this point, so can safely remove and stick with mypy defaults (which is more strict)

[codecov]

Codecov Report

Merging #27048 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #27048      +/-   ##
==========================================
- Coverage      92%      92%   -0.01%     
==========================================
  Files         180      180              
  Lines       50754    50754              
==========================================
- Hits        46698    46695       -3     
- Misses       4056     4059       +3

Flag	Coverage Δ
#multiple	90.64% <ø> (ø)	⬆️
#single	41.82% <ø> (-0.1%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️
pandas/core/frame.py	96.89% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.94% <0%> (+0.1%)	⬆️

---

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[codecov]

Codecov Report

Merging #27048 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #27048      +/-   ##
==========================================
- Coverage      92%      92%   -0.01%     
==========================================
  Files         180      180              
  Lines       50754    50754              
==========================================
- Hits        46698    46695       -3     
- Misses       4056     4059       +3

Flag	Coverage Δ
#multiple	90.64% <ø> (ø)	⬆️
#single	41.82% <ø> (-0.1%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️
pandas/core/frame.py	96.89% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.94% <0%> (+0.1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4aa1d6...db3d54d. Read the comment docs.

[jreback]

thanks @WillAyd