NEP29: Test PyGMT on NumPy 1.21 (#1355)
NEP29: Test PyGMT on NumPy 1.21 (#1355)

Bumps [numpy](https://github.com/numpy/numpy) from 1.20.3 to 1.21.0 released on 23 June 2021.
- [Release notes](https://github.com/numpy/numpy/releases)
- [Changelog](https://github.com/numpy/numpy/blob/v1.21.0/doc/changelog/1.21.0-changelog.rst)
- [Commits](https://github.com/GenericMappingTools/pygmt/pull/numpy/numpy@v1.20.3...v1.21.0)

This is in line with PyGMT's policy on NEP29 at https://www.pygmt.org/v0.4.0/maintenance.html#dependencies-policy, xref #1074.
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python