Allow building in bootstrap mode: without theano

Theano currently ftbfs (segv in tests), and the dependency is only Recommends
in binary packages, so allow building w/o it.

sympy.spec

@@ -1,6 +1,8 @@
+%bcond_with bootstrap
+
 Name:           sympy
 Version:        1.8
-Release:        2%{?dist}
+Release:        2%{?dist}%{?with_bootstrap:~bootstrap}
 Summary:        A Python library for symbolic mathematics
 License:        BSD
 URL:            http://sympy.org/
@@ -40,13 +42,16 @@ BuildRequires:  python3-numpy-f2py
 BuildRequires:  %{py3_dist scipy}
 BuildRequires:  %{py3_dist setuptools}
 BuildRequires:  %{py3_dist sphinx-math-dollar}
-BuildRequires:  %{py3_dist theano-pymc}
 BuildRequires:  %{py3_dist wurlitzer}
 BuildRequires:  tex(latex)
 BuildRequires:  tex-dvipng
 BuildRequires:  xorg-x11-fonts-Type1
 BuildRequires:  xorg-x11-server-Xvfb
 
+%if %{without bootstrap}
+BuildRequires:  %{py3_dist theano-pymc}
+%endif
+
 %global _description\
 SymPy aims to become a full-featured computer algebra system (CAS)\
 while keeping the code as simple as possible in order to be\