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

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[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/[email protected] 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]

LGTM.
cc @ZigaMr

[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 #355