Steps to reproduce the issue

The plotly package incorrectly uses one and only one version of it's dependencies specified in upstream's requirements.txt instead of specifying a minimum version like upstream's setup.py:
https://github.com/plotly/plotly.py/blob/v2.2.0/setup.py#L49

This is is introducing a circular dependency in my trying to use a fewer version of [email protected] which has a bugfix my labmate needs. I can't install a newer version of py-requests because py-plotly use a conflicting

$ spack spec py-deeptools
Input spec
--------------------------------
py-deeptools

Concretized
--------------------------------
==> Error: An unsatisfiable version constraint has been detected for spec:

    [email protected]:
        ^[email protected]:
            ^[email protected]:2.8,3.5:
                ^[email protected]:


while trying to concretize the partial spec:

    [email protected]:
        ^[email protected]:
            ^[email protected]:2.8,3.5:
                ^[email protected]:


py-plotly requires py-requests version 2.3.0, but spec asked for 2.5.0:

Information on your system

$ spack debug report
* **Spack:** 0.14.2
* **Python:** 3.6.9
* **Platform:** linux-ubuntu18.04-zen

# /etc/spack/packages.yaml 
packages:
  all:
    permissions:
      write: group
      group: users
  r:
    paths:
      [email protected]: /usr/bin
    buildable: false
  # py-deeptools:
  #   providers:
  #     'python@3'

# /etc/spack/repos.yaml 
repos:
- /var/spack/repos/corelab
- $spack/var/spack/repos/builtin

# /etc/spack/mirrors.yaml 
mirrors:
  local: file:///var/spack/mirror

$ tree -F /var/spack/mirror/
/var/spack/mirror/
├── jdk/
│   ├── jdk-11.0.4_10.tar.gz -> jdk-11.0.4_linux-x64_bin.tar.gz
│   └── jdk-11.0.4_linux-x64_bin.tar.gz
└── trf/
    └── trf409.linux64

2 directories, 3 files

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have run spack maintainers <name-of-the-package> and @mentioned any maintainers
• I have uploaded the build log and environment files
• I have searched the issues of this repo and believe this is not a duplicate