title: Anaconda 4.7.5 - Warning about conda-build <3.18.3 and issues with python packages
body: <p>I run the Anaconda python distribution on Ubuntu Linux 18.04 LTS x64 and just updated the distribution with the usual <code>conda update --all</code>. After that, the command line message suggested updating the conda base defaults. Now for some reason, I am having a couple of issues. First, I am unable to launch <code>jupyter lab</code>, even after trying to reinstall <code>conda install jupyter</code>. And second I am getting this new warning message.</p>
  
  <pre><code>    WARNING conda.base.context:use_only_tar_bz2(632):
   Conda is constrained to only using the old .tar.bz2
   file format because you have conda-build installed,
   and it is &lt;3.18.3.  Update or remove conda-build to
   get smaller downloads and faster extractions.
  </code></pre>
  
  <p>So I looked and found this <a href="https://www.anaconda.com/how-we-made-conda-faster-4-7/" rel="noreferrer">blog post</a> from today
  about making Anaconda faster. But the post seems to be more information and does not seem to recommend upgrading right away. </p>
  
  <p>Here is the output from <code>conda info</code>.</p>
  
  <pre><code>     active environment : XXX
      active env location : XXX
              shell level : 2
         user config file : ../.condarc
   populated config files : ../.condarc
            conda version : 4.7.5
      conda-build version : 3.17.8
           python version : 3.6.6.final.0
         virtual packages : __cuda=10.1
         base environment : ../anaconda3  (writable)
             channel URLs : https://repo.anaconda.com/pkgs/main/linux-64
                            https://repo.anaconda.com/pkgs/main/noarch
                            https://repo.anaconda.com/pkgs/r/linux-64
                            https://repo.anaconda.com/pkgs/r/noarch
            package cache : ../.conda/pkgs
         envs directories : ../.conda/envs
                 platform : linux-64
               user-agent : conda/4.7.5 requests/2.22.0 CPython/3.6.6 Linux/4.15.0-50-generic ubuntu/18.04.2 glibc/2.27
                  UID:GID : 1000:1000
               netrc file : None
             offline mode : False
  </code></pre>
  
  <p>Has anyone else run into this issue? Should I delete my old anaconda distribution and download and install the new version of 4.7, or is there a simpler fix?</p>
  
  <p><strong>UPDATE</strong></p>
  
  <p>So I did post this issue to the <code>conda</code> repo on Github. The current issue open against this problem is listed below. According to <code>msarahan</code> here is the basis of the problem and just a temporary workaround </p>
  
  <p><a href="https://github.com/conda/conda/issues/8842" rel="noreferrer">https://github.com/conda/conda/issues/8842</a></p>
  
  <blockquote>
    <ol>
    <li><p>anaconda is a meta-package. Each version consists of a set of versions
    that have all gone through QA together as a set. If you change any
    version of any package in that collection, you no longer have that
    metapackage, because you have strayed from that known set. There is a
    special version of that metapackage, custom, that is meant to handle
    this relaxation of constraints. The "custom" version depends only on a
    particular version of python - it removes the constraints on all other
    packages. </p></li>
    <li><p>conda 4.7 builds up its candidates for addition differently
    from earlier conda versions. It starts with specs from the history,
    and tries to constrain things where it can, to speed up the solution.
    When conda finds the anaconda metapackage with the "custom" version,
    it keeps it, but all of those other dependencies are now orphaned.
    This is why conda is removing them - they have no spec in the history
    that tells conda to keep them. </p></li>
    </ol>
    
    <p>You can restore these by running <code>conda
    install --only-deps anaconda</code>. From then on, all of those packages are
    considered part of your explicit history, and you won't have further
    problems like this.</p>
  </blockquote>