evaluate env markers for requirements

[grische]

This allows specifying requirements with env markers to be evaluated for
the actually defined envs.

For example, this file with with both Python 3.13+3.7 will pull in importlib-metadata:

[env.linux-py313]
os_name = posix
sys_platform = linux
platform_machine = x86_64
platform_python_implementation = CPython
platform_system = Linux
python_version = 3.13
python_full_version = 3.13.5
implementation_name = cpython

[env.linux-py37]
os_name = posix
sys_platform = linux
platform_machine = x86_64
platform_python_implementation = CPython
platform_system = Linux
python_version = 3.7
python_full_version = 3.7.1
implementation_name = cpython

[requirements]
importlib-metadata = ~=4.5.0; python_version < "3.8"

and once you drop the linux-py37 env, it will stop mirroring it without having to adjust the requirements list as well:

[env.linux-py313]
os_name = posix
sys_platform = linux
platform_machine = x86_64
platform_python_implementation = CPython
platform_system = Linux
python_version = 3.13
python_full_version = 3.13.5
implementation_name = cpython

[requirements]
importlib-metadata = ~=4.5.0; python_version < "3.8"