Odin casts the following spell on Mjolnir:
Whosoever holds this hammer, if he be worthy, shall possess the power of Thor.
I have boldfaced he to emphasize that Odin didn't say the following.
Whosoever holds this hammer, if they be worthy, shall possess the power of Thor.
[minor but important digression: English does have a use of they as a singular, gender-neutral pronoun, so this is is a valid thing to say]
These two spells are crucially different. By using he instead of they, Odin is effectively restricting the use of Mjolnir to men. The Black Widow and Maria Hill could be worthier than anybody else, but they would still not possess the power of Thor. Is there any in-universe evidence that things work this way?