Fix get_pragma_spec() to exclude commented out pragma statements on compile

[freddie71010]

Fix get_pragma_spec() to exclude commented out pragma statements at the beginning of the file.

What I did

Refactor brownie/project/sources.py::get_pragma_spec()

How I did it

Reconfigure regex to grab all pragma solidity statements in the file and then loop through and skip over any that are prefixed with a "//". Continue looping until first valid pragma statement is found.

How to verify it

Wrote three new unit tests to look at scenarios where:

• one valid pragma version statement listed
• no valid pragma version statement listed
• one valid in-between pragma version statement listed

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation
• I have added an entry to the changelog