Support using commands not named gpg for signing

Fixes #29
Restrict requests to at least 2.0
Print dependency versions with twine --version
Merge pull request #81 from sigmavirus24/bug/min-requests-version

Restrict requests to at least 2.0
Add tests and refactor upload function
Merge pull request #82 from sigmavirus24/feature/show-dependency-versions

Print dependency versions with twine --version
Merge pull request #79 from sigmavirus24/feature/29

Support using commands not named gpg for signing
Read deprecated style of PyPI rc files

Fixes #61
Merge pull request #83 from pypa/bug/61

Read deprecated style of PyPI rc files