Update requirements.txt

[Redsandro]

It appears SciPy 1.7.1 was retracted from pip. Bugfix release 1.7.2 is the closest available release. Bumping so builds will no longer fail.

ERROR: Could not find a version that satisfies the requirement scipy==1.7.1 (from versions: 0.8.0, 0.9.0, 0.10.0, 0.10.1, 0.11.0, 0.12.0, 0.12.1, 0.13.0, 0.13.1, 0.13.2, 0.13.3, 0.14.0, 0.14.1, 0.15.0, 0.15.1, 0.16.0, 0.16.1, 0.17.0, 0.17.1, 0.18.0, 0.18.1, 0.19.0, 0.19.1, 1.0.0, 1.0.1, 1.1.0, 1.2.0, 1.2.1, 1.2.2, 1.2.3, 1.3.0rc1, 1.3.0rc2, 1.3.0, 1.3.1, 1.3.2, 1.3.3, 1.4.0rc1, 1.4.0rc2, 1.4.0, 1.4.1, 1.5.0rc1, 1.5.0rc2, 1.5.0, 1.5.1, 1.5.2, 1.5.3, 1.5.4, 1.6.0rc1, 1.6.0rc2, 1.6.0, 1.6.1, 1.7.2, 1.7.3, 1.8.0rc1, 1.8.0rc2, 1.8.0rc3, 1.8.0rc4, 1.8.0, 1.8.1, 1.9.0rc1, 1.9.0rc2, 1.9.0rc3, 1.9.0)
ERROR: No matching distribution found for scipy==1.7.1

[MerlijnWajer]

Thanks!