consider switching from toml to tomli

[graingert]

Is your feature request related to a problem? Please describe.
pip is moving from toml to tomli pypa/pip#10035 coveragepy should consider following suit

• pip: Replace vendored toml with tomli for TOML v1.0.0 compat pypa/pip#10035
• build: Consider switching to TOML v1 compliant parser pypa/build#308
• black: Switch toml TOML library for tomli psf/black#2301
• pytest: Support TOML v1.0.0 syntax in pyproject.toml pytest-dev/pytest#8857
• coveragepy: Support TOML v1.0.0 syntax in pyproject.toml #1186
• setuptools_scm fix #585: switch to tomli pypa/setuptools-scm#586
• towncier swap toml with tomli twisted/towncrier#354

[graingert]

pytest has now switched to tomli, meaning that installing pytest-cov requires toml and tomli