MAINT: Fix failing tests, pin XGBoost in test dependencies to 1.7.6

[connortann]

Overview

Closes #3254

Pins XGBoost in test dependencies to 1.7.6.

Context

The baseline images we have are signifcantly altered in XGBoost 2.0.0. So, pinning the test dependency to the last working version seems like a pragmatic short-term fix to get the tests working again.

Once the tests are fixed, we should consider:

• Can we get the tests passing without needing this pin? ENH: Test against XGBoost 2.0 #3256
• How to manage test dependencies. Should they all be pinned?

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +17.44% 🎉

Comparison is base (e8a176e) 40.16% compared to head (d43698e) 57.61%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3255       +/-   ##
===========================================
+ Coverage   40.16%   57.61%   +17.44%     
===========================================
  Files          88       88               
  Lines       12510    12511        +1     
===========================================
+ Hits         5025     7208     +2183     
+ Misses       7485     5303     -2182     

see 33 files with indirect coverage changes

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[connortann]

@dsgibbons , @thatlittleboy , would you kindly take a look and review? It would be good fix the tests, so other PRs are not blocked.

[dsgibbons]

LGTM, seems like a sensible fix for now