Lazy pandas import to improve startup time

[janosh]

Import times profiled and reported by @mturiansky in materialsproject/pymatgen#3563 (comment).

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (5d37679) 78.82% compared to head (127133c) 78.57%.

Files	Patch %	Lines
monty/json.py	68.75%	4 Missing and 1 partial ⚠️
monty/os/__init__.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
- Coverage   78.82%   78.57%   -0.26%     
==========================================
  Files          27       27              
  Lines        1393     1400       +7     
  Branches      297      297              
==========================================
+ Hits         1098     1100       +2     
- Misses        239      244       +5     
  Partials       56       56              

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