CI: On Azure, pin pytest-xdist to version 2.5.0
CI: On Azure, pin pytest-xdist to version 2.5.0

The Azure job "Main prerelease_deps_coverage_64bit_blas" is
failing because of an incompatibility between pytest-cov and
pytest-xdist 3.0.2, so use pytest-xdist 2.5.0 until the issue
https://github.com/pytest-dev/pytest-cov/issues/557
is resolved.

[skip cirrus] [skip actions] [skip circle]

scipy/scipy,WarrenWeckesser/scipy,scipy/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,andyfaff/scipy,WarrenWeckesser/scipy,scipy/scipy,andyfaff/scipy,andyfaff/scipy,Stefan-Endres/scipy,Stefan-Endres/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,scipy/scipy,WarrenWeckesser/scipy,andyfaff/scipy,andyfaff/scipy,Stefan-Endres/scipy,Stefan-Endres/scipy,andyfaff/scipy,scipy/scipy,Stefan-Endres/scipy,scipy/scipy