DOC: increase recursion limit on sphinx builds

[jreback]

xref #15637 (comment)

hopefully should eliminate the non-deterministic pickle errors in doc-building :<

[codecov-io]

Codecov Report

Merging #15641 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15641      +/-   ##
==========================================
- Coverage   91.02%      91%   -0.03%     
==========================================
  Files         143      143              
  Lines       49324    49324              
==========================================
- Hits        44898    44887      -11     
- Misses       4426     4437      +11

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	91.76% <0%> (-0.35%)	❌
pandas/core/frame.py	97.87% <0%> (-0.1%)	❌

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[jorisvandenbossche]

Great! That was indeed the error I saw locally (so not related to our pickle changes). I just didn't see it anymore afterwards, and on travis in the PR, but apparently it is not each time, or so.