contracts: silence SIWE Solidity tests compile warnings

[CedarMist]

• Updates clients/py requirements to specify Web3.py v6.x
• Fixes warnings in contracts/tests/SiweParserTests.sol

[netlify[bot]]

✅ Deploy Preview for oasisprotocol-sapphire-paratime canceled.

Name	Link
🔨 Latest commit	bde8bfac438c0cfec1c51467901c3fe9f8728d7d
🔍 Latest deploy log	https://app.netlify.com/sites/oasisprotocol-sapphire-paratime/deploys/66cc8a2f0f15f000084b4766

[CedarMist]

Apparently the Python client isn't compatible with Web3.py 7.x and we're not pinning the version in requirements.txt

7.x was released Aug 21, prior to that 6.x was being used.

---

Silences these warnings:

> @oasisprotocol/sapphire-contracts@0.2.10 build /src/sapphire-paratime/contracts
> hardhat compile

Warning: Function state mutability can be restricted to pure
 --> contracts/tests/SiweParserTests.sol:7:5:
  |
7 |     function testHexStringToAddress(bytes memory addr)
  |     ^ (Relevant source part starts here and spans across multiple lines).

Warning: Function state mutability can be restricted to pure
  --> contracts/tests/SiweParserTests.sol:15:5:
   |
15 |     function testFromHexChar(uint8 c) external view returns (uint8) {
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning: Function state mutability can be restricted to pure
  --> contracts/tests/SiweParserTests.sol:19:5:
   |
19 |     function testSubstr(
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning: Function state mutability can be restricted to pure
  --> contracts/tests/SiweParserTests.sol:27:5:
   |
27 |     function testParseUint(bytes memory b) external view returns (uint256) {
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning: Function state mutability can be restricted to pure
  --> contracts/tests/SiweParserTests.sol:31:5:
   |
31 |     function testParseField(
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning: Function state mutability can be restricted to pure
  --> contracts/tests/SiweParserTests.sol:39:5:
   |
39 |     function testParseArray(bytes calldata str, uint256 i)
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning: Function state mutability can be restricted to pure
  --> contracts/tests/SiweParserTests.sol:47:5:
   |
47 |     function testParseSiweMsg(bytes calldata siweMsg)
   |     ^ (Relevant source part starts here and spans across multiple lines).

[matevz]

Apparently the Python client isn't compatible with Web3.py 7.x and we're not pinning the version in requirements.txt

7.x was released Aug 21, prior to that 6.x was being used.

Related https://github.com/oasisprotocol/sapphire-paratime/issues/355