Fix function state mutability warning

[zemse]

PR #1739 hardcodes ERC777 granularity to 1, this causes the solc compiler generates the following warning:

WARNING
  @openzeppelin/contracts/token/ERC777/ERC777.sol:116:5: Warning: Function state mutability can be restricted to pure
      function granularity() public view override returns (uint256) {
      ^ (Relevant source part starts here and spans across multiple lines).

This PR fixes the above warning by changing the state mutability of granularity function from view to pure.

[zemse]

This gives error in 0.6, but in the newly released 0.7 first language feature, it is mentioned that pure can override view. Source: https://github.com/ethereum/solidity/releases/tag/v0.7.0

As seen the linter test is failing, looks like it also needs to be upgraded. I'll take a look if I can do that.

Edit: I tried upgrading solhint to 3.1.0 (from 3.0.0), but seems like the version in which the constructor visibility issue is fixed would be released after protofire/solhint#241 gets merged. I think this branch already has the lint CI failing (since this PR changes function state mutability while lint error is regarding constructor visibility), so marking this ready.

[frangio]

Awesome @zemse! Thank you.

[fvictorio]

I'm about to publish a version of solhint with support for 0.7. Should I test it in the OpenZeppelin:solc-0.7 branch?

[frangio]

@fvictorio Yes, please! Let me know how it goes.

[fvictorio]

@frangio Just published solhint@3.2.0, that works fine in that branch (there's a bazillion warnings though). I tried to make a PR upgrading it but node-gyp is messing with me 😢

[frangio]

@fvictorio Thanks! Seems to work well. Just updated our 0.7 branch.