cleanup: fix typings for context.py

[shubhbapna]

part of #226

Fixes the following type errors:

src/fromager/context.py:54: error: Need type annotation for "_build_stack" (hint: "_build_stack: list[<type>] = ...")  [var-annotated]
src/fromager/context.py:55: error: Need type annotation for "_build_requirements" (hint: "_build_requirements: set[<type>] = ...")  [var-annotated]
src/fromager/context.py:62: error: Need type annotation for "_seen_requirements" (hint: "_seen_requirements: set[<type>] = ...")  [var-annotated]
src/fromager/context.py:69: error: List item 1 has incompatible type "str | None"; expected "str"  [list-item]
src/fromager/context.py:72: error: Missing type parameters for generic type "tuple"  [type-arg]
src/fromager/context.py:75: error: Function is missing a return type annotation  [no-untyped-def]
src/fromager/context.py:80: error: Missing type parameters for generic type "tuple"  [type-arg]
src/fromager/context.py:81: error: Incompatible return value type (got "bool", expected "tuple[Any, ...]")  [return-value]
src/fromager/context.py:83: error: Function is missing a return type annotation  [no-untyped-def]
src/fromager/context.py:88: error: Missing type parameters for generic type "list"  [type-arg]
src/fromager/context.py:125: error: Function is missing a return type annotation  [no-untyped-def]
src/fromager/context.py:125: note: Use "-> None" if function does not return a value

[dhellmann]

This looks OK to me, but @tiran is the type annotation expert so I'll let him review, too.