Consider prepending '=' to install version when no character provided

[nbigaouette]

As reported in #100, confusion can happen when a version to install is written without any leading character.

For example:

pycors install 3.8.0

will be interpreted as ^3.8.0 by the semver crate. This will in turn attempt to install a version matching >=3.8.0,<4.0.0, which at the current time matches 3.9.0.

While "valid" according to semver, it's probably not what the user expected.

Prepending a = in front of a requested install when no character is present should be considered. The downside of this is that the string provided is not strictly semver anymore.

[nbigaouette]

Some usefull notes regarding semantic versioning:

https://stackoverflow.com/questions/54720072/dependency-version-syntax-for-python-poetry