CI: Unpin 3.7.3 pins
Seen in at least two builds: https://travis-ci.org/pandas-dev/pandas/jobs/570645656

```
$ ci/run_tests.sh
2333pytest -m "single and (not slow and not network)" -n 1 -s --strict --durations=10 --junitxml=test-data-single.xml pandas
2334ImportError while loading conftest '/home/travis/build/pandas-dev/pandas/pandas/conftest.py'.
2335pandas/__init__.py:38: in <module>
2336    "the C extensions first.".format(module)
2337E   ImportError: C extension: No module named 'pandas._libs.tslibs.conversion' not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.
2338The command "ci/run_tests.sh" exited with 4.
2339
```

Fixing in https://github.com/pandas-dev/pandas/pull/27868

Leaving this open to revert.
