Using RMT with Mercurial and Simple version generation is broken when Mercurial version > 3

Nobody has claimed this yet.

It seems that beginning with Mercurial version 3.0, it is now impossible to use purely numeric tag name :

```
$ hg tag 1
abort: cannot use an integer as a name
$ hg --version
Mercurial Distributed SCM (version 3.1)
```

But it is working on version 2.2 :

```
$ hg tag 1
$ hg --version
Mercurial Distributed SCM (version 2.2.2)
```

I don't know if there is a lot of Mercurial users out there using RMT, but this issue should be addressed. I see two possible solutions :
1. Forbid the use the Simple version generation when Mercurial is used
2. Add some kind of prefix to simple tags, like 'v\d+' instead of the current '\d+'

The problem with 2) is that it is can be a big BC break depending on the implementation.

FYI : the test suite is currently failing when ran with the "faulty" Mercurial version.

Start by running the test suite with Mercurial 3.1 and inspect the Simple version generation and Mercurial integration involved in creating numeric tags. Determine how the incompatibility should be handled without an unintended backward-compatibility break, then confirm the chosen behavior with the failing suite.

Written by the indexing model from the issue text.