Bump version to 0.6.0

[oscarbenjamin]

Ready for a new release.

Changes since 0.5.0:

• gh-112,
  gh-111,
  gh-110,
  gh-108:
  Add pyproject.toml and build dependencies. This means that python-flint can
  be built from source without --no-build-isolation.
• gh-109:
  Use exact division for non-field domains. Now fmpz(6)/fmpz(3) returns an
  exact result fmpz(2) or raises an error if an exact result is not possible.
  Similar changes for fmpz_poly/fmpz, fmpz_mat/fmpz, and for polynomial
  division with fmpz_poly, fmpq_poly, nmod_poly and fmpz_mod_poly.
• gh-106:
  Add fmpz_mod_mat for matrices of integers mod n where n is larger than
  word sized.
• gh-104:
  Bump Flint from 3.0.0 to 3.0.1