Getting the wrong version of pandas when installing from requirements.txt
#### Code Sample, a copy-pastable example if possible

```
$ python3 -m pip install --user -r requirements.txt | grep -i pandas | grep -vi 'Installing collected packages'
below cmd output started 2020 Sat Feb 29 03:20:45 AM UTC
Collecting pandas==0.23.4 (from -r requirements.txt (line 43))
  Using cached https://files.pythonhosted.org/packages/e1/d8/feeb346d41f181e83fba45224ab14a8d8af019b48af742e047f3845d8cff/pandas-0.23.4-cp36-cp36m-manylinux1_x86_64.whl
above cmd output done    2020 Sat Feb 29 03:21:55 AM UTC
dstromberg@TransientTracking-2:~/src/grok/just-master x86_64-pc-linux-gnu 126478

$ python3 -m pip freeze | grep pandas
below cmd output started 2020 Sat Feb 29 03:22:46 AM UTC
pandas==1.0.1
above cmd output done    2020 Sat Feb 29 03:22:47 AM UTC
dstromberg@TransientTracking-2:~/src/grok/just-master x86_64-pc-linux-gnu 126478

$ grep pandas requirements.txt 
below cmd output started 2020 Sat Feb 29 03:22:59 AM UTC
#pandas
pandas==0.23.4
above cmd output done    2020 Sat Feb 29 03:22:59 AM UTC
dstromberg@TransientTracking-2:~/src/grok/just-master x86_64-pc-linux-gnu 126478

```

#### Problem description

I'm looking at a large codebase that needs an old version of pandas - but for some reason pip install isn't giving it to me.

If I delete pandas and install it manually with the following, I get the right version of pandas.  There's something about requirements.txt that isn't quite right.

```
$ python3 -m pip install pandas==0.23.4 | grep -i pandas | grep -v 'Installing collected packages'
below cmd output started 2020 Sat Feb 29 03:30:27 AM UTC
Collecting pandas==0.23.4
  Using cached https://files.pythonhosted.org/packages/e1/d8/feeb346d41f181e83fba45224ab14a8d8af019b48af742e047f3845d8cff/pandas-0.23.4-cp36-cp36m-manylinux1_x86_64.whl
above cmd output done    2020 Sat Feb 29 03:30:35 AM UTC
dstromberg@TransientTracking-2:~/src/grok/just-master x86_64-pc-linux-gnu 126478

$ python3 -m pip freeze | grep pandas
below cmd output started 2020 Sat Feb 29 03:30:47 AM UTC
pandas==0.23.4
above cmd output done    2020 Sat Feb 29 03:30:48 AM UTC
dstromberg@TransientTracking-2:~/src/grok/just-master x86_64-pc-linux-gnu 126478

```

Any suggestions?

Thanks for the cool software :)
